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SUMMARY (CONTINUED) OF THE GEOLOGY OF THE MOUNT BAKER 30-MINUTE BY 60-MINUTE QUADRANGLE, WASHINGTON
Rocks west of the Straight Creek Fault: the Northwest Cascades SystemWest of the Straight Creek Fault, the North Cascades appear to be composed of two fundamental regional structural blocks separated by a complex tectonic belt and high-angle faults (figs. 1, 4; see also Tabor and others, 1989; Tabor 1994). The northeastern structural block, exposed primarily in the Mount Baker quadrangle, is mostly composed of Paleozoic and Mesozoic volcanic arc and associated clastic wedge deposits along with more thoroughly metamorphosed oceanic rocks, thrust in the mid-Cretaceous into a series of nappes. The overall structure has been likened to a regional mélange by Brown (1987) who, modifying the earlier terminology of Misch (1966, p. 128), called rocks of this structural block the Northwest Cascade System. As explained below, we now think that the structure displays more order than regional mélange implies. The southwestern block, exposed just south of the Mount Baker quadrangle, is mostly Mesozoic clastic rocks of submarine-fan origin and relatively unmetamorphosed oceanic rocks. Tabor and others (1989, 1993), Frizzell and others (1987), and Tabor (1994) described this block as the western and eastern mélange belts.
Four major nappes, stacked along folded thrusts, and their probably autochthonous footwall make up the Northwest Cascades System (fig. 4). The structural stratigraphy of the Northwest Cascades System appears to be consistent over a wide area of northwest Washington. The rocks in the three lowermost nappes and the autochthon differ enough in lithology, structure, and metamorphic history to warrant consideration as separate terranes (Tabor and others, 1989; Haugerud and others, 1994), but the highest and youngest Gold Run Pass Nappe consists of slices of the lower nappes and autochthon.
The stacked nappes have been displaced by extensional faults. The Glacier Extensional Fault is a regional structure that cuts out the nappe stratigraphy along the west side of the Mount Baker quadrangle.
Rocks of the autochthon
Nooksack Formation (new name adopted here)
At the bottom of the exposed stack of nappes is the Middle Jurassic Early Cretaceous Nooksack Formation consisting of marine clastic rocks overlying and interfingering stratigraphically with the Middle Jurassic Wells Creek Volcanic Member (newly adopted herein). Sondergaard (1979) considered the rocks of the Nooksack Formation to be a submarine fan deposit associated with a volcanic arc. The Nooksack is generally not strongly penetratively deformed and recrystallized, although in many areas it has slaty cleavage and has been partially recrystallized in sub-greenschist facies.
Excelsior and Welker Peak Nappes
Chilliwack Group of Cairnes (1944) and Cultus Formation of Brown and others (1987)
Structurally overlying the Nooksack Formation along the Excelsior Ridge Thrust Fault (the Church Mountain thrust of Misch, 1966; see Table 1) is the Chilliwack Group of Cairnes (1944), composed of partly metamorphosed basaltic and andesitic volcanic rocks, sandstone, siltstone, shale, and minor limestone. The rocks of the Chilliwack formed in an arc setting, and marble in the unit yields fossils ranging in age from Silurian (?) and Devonian to Permian, though most are Mississippian. Rocks are slaty to phyllitic, and planar structures are commonly low-angle. Lawsonite and aragonite are common metamorphic minerals, but generally are very fine grained. The Chilliwack is depositionally overlain by the Cultus Formation of Brown and others (1987), a Triassic to Early Jurassic marine and dacitic volcanic unit. The Chilliwack Group and Cultus Formation occur mainly in the Excelsior Nappe (fig. 5). The Excelsior Nappe contains significant internal thrusts; rocks of the Chilliwack Group and Cultus Formation are regionally overturned, and they have penetrative fabrics in most locales, suggesting a pre-mid Cretaceous, possible pre-Late Jurassic tectonic event not seen in the underlying Nooksack Formation.
A unit with similarities to the clastic facies of the Chilliwack Group, as well as other units in Northwest Cascade System is the slate of Rinker Ridge. It is poorly exposed in the lower Skagit River valley of the Mount Baker quadrangle. Good exposures in the Sauk River quadrangle to the south (fig. 1; Tabor and others, in press) indicate that the slate of Rinker Ridge appears to be a fault bounded block within extensive outcrops of the Easton Metamorphic Suite. Tabor and others (in press) discuss the possible protoliths for the slate of Rinker Ridge and tentatively assign it to the Chilliwack Group. We include it in the Excelsior Nappe in figure 4.
Bell Pass mélange
The Chilliwack Group and Cultus Formation are overlain along the Welker Peak Thrust Fault by the Bell Pass mélange, much of which is composed of the Elbow Lake Formation of Brown and others (1987), a mixed assemblage of foliated sandstone, argillite (phyllite), ribbon chert, basalt, and very rare marble. Commonly in or associated with the Elbow Lake Formation are ultramafic rocks, various blocks of gneiss and schist, and granitoid rocks ranging from granite to gabbro in composition. These are locally mapped as the Twin Sisters Dunite of Ragan (1963), the blueschist of Baker Lake, the Vedder Complex of Armstrong and others (1983), and the Yellow Aster Complex of Misch (1966). Ages of radiolarians from chert blocks in the Elbow Lake Formation range from Pennsylvanian to Jurassic. Gneiss and schist of the Vedder Complex yield K-Ar and Rb-Sr ages indicating Permian metamorphism. Zircons from Yellow Aster paragneiss have discordant U-Pb ages interpreted to be Precambrian and probably representing detritus derived from Proterozoic basement. Zircons from orthogneiss in the complex yield middle Paleozoic ages. These rocks make up the Welker Peak Nappe (figs. 4, 5). In part, the Bell Pass mélange is coincident physically and in concept with the thick tectonic zone at the base of the mid-Cretaceous Shuksan Thrust Fault as described by Misch (1966, 1980), which separates more thoroughly metamorphosed rocks of the Easton Metamorphic Suite equivalent to Misch's (1966) Shuksan Metamorphic Suite from the structurally underlying Nooksack Formation, Chilliwack Group, and Cultus Formation. However, we suspect that some of the mixing and deformation within the Bell Pass mélange predates mid-Cretaceous tectonism and is unrelated to the Shuksan Thrust Fault (see description of the Bell Pass mélange in I-2660).
Easton Metamorphic Suite
The Easton Metamorphic Suite, also referred to the Easton terrane (Tabor and others, 1989), is composed of the Shuksan Greenschist and the Darrington Phyllite. It generally overlies lower nappes along the Shuksan Thrust Fault. The Easton records a more thorough episode of high P/T metamorphism than the other units in the Northwest Cascades System.
The well-recrystallized Shuksan Greenschist and Darrington Phyllite were metamorphosed in blueschist facies, with blue amphibole and lawsonite in rocks of appropriate composition. Chemical composition of most Shuksan Greenschist indicates it was derived from mid-ocean ridge basalt (MORB). Based on isotopic analyses, Armstrong (1980) and Brown and others (1982) interpret the Easton to have a Middle to Late Jurassic depositional age (about 150 160 Ma) and an Early Cretaceous metamorphic age (about 120 130 Ma), with evidence for local earlier metamorphism.
The semischist and phyllite of Mount Josephine crops out in extensive tracts along the west side of the Mount Baker quadrangle and farther west. This unit overlies the Bell Pass mélange along a thrust here correlated with the Shuksan Thrust Fault. Rocks of this unit are similar lithologically to the Darrington Phyllite, but differ in that their protolith was sandier and that they appear less thoroughly recrystallized than the Darrington. Previous workers have included this unit in the Darrington Phyllite. Greenschist and blueschist intercalations are lacking and rare metavolcanic rocks are greenstone.
ROCKS BETWEEN THE STRAIGHT CREEK FAULT AND THE ROSS LAKE FAULT ZONE
In the Mount Baker quadrangle, the high-grade metamorphic core of the North Cascades (fig. 1) is made up of the Chelan Mountains terrane and plutons that intrude it, as well as the Skagit Gneiss Complex, derived from the supracrustal rocks of the Chelan Mountains terrane by higher-grade metamorphism and pervasive deep-seated intrusion. K-AR ages of schists and gneisses in much of the region between the Straight Creek Fault and the Ross Lake Fault Zone are almost all middle and late Eocene, reflecting early Tertiary unroofing and cooling. Much of the core has been intruded by arc-root magmas of the Tertiary Chilliwack composite batholith.
Rocks of the Chelan Mountains terrane include the Napeequa Schist, the metaplutonic rocks of the Marblemount Dumbell belt (fig. 1), and the Cascade River Schist. Rocks of the Napeequa Schist are mostly micaceous quartzite, fine-grained hornblende schist, and amphibolites derived from a protolith of oceanic chert and basalt. Minor marble and small bodies of metamorphosed ultramafic rock are also characteristic.
Protolith of the Cascade River Schist was a thick sequence of arc-derived clastic rocks with minor volcanic rocks, now metamorphosed to plagioclase-rich mica schist, metaconglomerate, and amphibolitic schist. Prominent in the metaconglomerate are clasts of the Marblemount pluton, indicating that the Cascade River protolith was deposited on or near eroded Marblemount pluton. Minor constituents of the Cascade River Schist are silicic schists (metatuff), marble, and amphibolite. U-Pb analysis of zircons from a dacitic metatuff yielded ages of about 220 Ma, that is, Late Triassic.
The Marblemount pluton comprises the northern end of the Marblemount Dumbell plutonic belt which stretches about 75 km southeast from the Mount Baker quadrangle (fig. 1). The crystallization age of the Marblemount protolith is also 220 Ma. That the Cascade River Schist and the Marblemount pluton are the same age suggests deposition of the Cascade River Schist protolith in a forearc or intra-arc basin wherein intrusion of arc-root plutons, such as the Marblemount pluton, was followed by rapid unroofing and further deposition of arc volcanic rocks.
Large amounts of tonalitic to granodioritic magma intruded the supracrustal rocks of the Chelan Mountains terrane in Late Cretaceous and earliest Tertiary time. These stitching plutons were deformed and partially recrystallized to orthogneisses, a process that began in the Late Cretaceous and continued into the early Tertiary as shown by Eocene K-AR ages and fabrics similar to demonstrably Eocene fabrics in the nearby Skagit Gneiss Complex. South of the Mount Baker quadrangle, the plutons can be grouped by mineral composition, d18O content, and some structural features into two groups, which in general reflect modal compositions.
Within a tonalitic group, the orthogneisses of Haystack Creek and Mount Triumph are lithologically similar to orthogneiss bodies within the Skagit Gneiss Complex (see below) which have 60 70 Ma U-Pb zircon ages.
Within a granodioritic group: the Eldorado Orthogneiss was intruded at 90 Ma and is strongly deformed and extensively recrystallized; the orthogneiss of Marble Creek was intruded at about 75 Ma and is extensively deformed and recrystallized; the Hidden Lake stock, apparently on the edge of the deep and thoroughly metamorphosed orogen, was intruded at 75 Ma and is well recrystallized but less deformed than the tonalitic bodies mentioned above; the orthogneiss of Alma Creek is even less deformed and perhaps slightly younger
The Skagit Gneiss Complex (Skagit Gneiss of Misch, 1966) is banded biotite gneiss, banded amphibolitic gneiss and large bodies of tonalitic orthogneiss, all mostly migmatitic. The banded gneisses contain abundant orthogneiss layers on all scales. Small bodies of mafic gneiss, mafic migmatite, ultramafic rock and marble crop out also. All of the complex is pervaded by concordant to discordant deformed bodies of light-colored tonalite and tonalitic pegmatite. Based on composition and observed transition to the protoliths, the banded gneisses appear to be highly metamorphosed Cascade River and Napeequa Schists. The orthogneiss of The Needle yields discordant U-Pb zircon ages and has textural evidence of multiple deformation, suggesting it is a highly metamorphosed pluton of the Late Triassic Marblemount intrusive episode. Much of the Skagit is permeated by dikes and irregular bodies of granite and, locally, granitic pegmatite which are characterized by a prominent lineation and weak, or absent, foliation; isotopic ages of the granites indicate middle Eocene intrusion.
ROCKS IN THE ROSS LAKE FAULT ZONE
Regionally the northwest-trending Ross Lake Fault Zone juxtaposes the higher-grade North Cascade core rocks with a little-metamorphosed sequence of Mesozoic marine and terrestrial deposits of the Methow terrane to the east (fig. 1). In the Mount Baker quadrangle several faults in the zone separate higher-grade metamorphic core rocks from a sliver of lower-grade schist and phyllite the rocks of Little Jack Mountain and a tract of minimally metamorphosed Late Paleozoic and Mesozoic oceanic rocks the Hozomeen Group. For much of their contact, the rocks of the Hozomeen Group overlie the Little Jack terrane along a low-angle thrust which probably predates the high-angle faults of the Ross Lake Fault Zone.
Within the Ross Lake Fault Zone, a group of plutons ranging from gabbro to granodiorite in composition and intruding rocks of the Little Jack terrane and of the Skymo Complex of Wallace (1976) makes up the Ruby Creek Heterogeneous Plutonic Belt of Misch (1966). Gneissic and massive plutons suggest a long history of intrusion during and after deformation in the Ross Lake Fault Zone. The only age available from the Ruby Creek belt is middle Eocene (Miller and others, 1989), from a body that on structural grounds must be among the youngest components of the belt.
The Skymo Complex of Wallace (1976), of unknown protolith age and terrane affinity, consists of locally orthopyroxene-bearing mafic to ultramafic cumulate igneous rocks intruded by clinopyroxene gabbro. Hyatt and others (1996) and Whitney and others (1996) suggest that the petrologic history of the Skymo Complex is unlike any other unit in the North Cascades. The unit is faulted against the phyllite and schist of Little Jack Mountain, in part along low-angle faults. Skymo rocks are also faulted against orthogneiss of the Skagit Gneiss Complex on the west, but are partially engulfed in tonalitic material associated with the metamorphism affecting the Skagit, suggesting that the faults separating the two units have had only modest displacements since Late Cretaceous metamorphism.
Following the example of McTaggart and Thompson (1967) we have, in reconnaissance, roughly subdivided the Hozomeen Group (newly adopted name) into: a lowermost exposed unit of probable upper Paleozoic greenstone with minor chert and limestone; a middle unit of predominantly Middle and Late Triassic ribbon chert and argillite; and an upper unit of predominantly Late Triassic greenstone, clastic sedimentary rocks, ribbon chert, and limestone, with minor Jurassic chert and clastic sedimentary rocks. These three units appear to correlate with the upper three of McTaggart and Thompson's four units.
Phyllite and Schist of Little Jack Mountain
Phyllite and schist of Little Jack Mountain comprise mostly biotite+amphibole-bearing metapelite and lesser meta-arenite, with minor fine-grained amphibolite and rare recrystallized ribbon chert and marble. Scattered pods of meta-ultramafic rocks are characteristic of the unit. Metadacite porphyry dikes, some with little deformation and others strongly lineated and (or) foliated, are abundant. The protolith age is prelate Cretaceous but otherwise unknown; we tentatively consider it to be Mesozoic, the age of most dominantly clastic terranes in the Pacific Northwest.
ROCKS EAST OF THE ROSS LAKE FAULT ZONE
A small area of sandstone and argillite, probably correlative with the Thunder Lake unit (O'Brien, 1986) exposed to the north in British Columbia, crops out on the east side of the quadrangle. These rocks are in the Methow terrane (fig. 1) and are separated from the Hozomeen Group by the Hozomeen Fault.
LATE AND POST OROGENIC DEPOSITS
Eocene extension associated with strike-slip faulting, opened depressions at shallow levels, where fluviatile feldspathic sandstone and conglomerate accumulated (Tabor and others, 1984; Johnson, 1985; Heller and others, 1987), while metamorphism continued in the Skagit Gneiss Complex. Most of such deposits are preserved outside of the Mount Baker quadrangle, but a few remnants crop out in the quadrangle. Sandstone and conglomerate of the Eocene Chuckanut Formation crop out along the west side of the quadrangle, in part separated from underlying older rocks by low-angle extensional faults. Smaller patches of probably partly correlative rocks (mapped as older sandstone and conglomerate) are preserved on Mount Despair, near Bacon Peak, under and near the volcanic rocks of Big Bosom Buttes, and along the Straight Creek Fault. Younger sandstone and conglomerate crops out along the Straight Creek Fault north of Marblemount where a clast of Marblemount pluton with a zircon fission track age of 45 Ma shows the deposit to be late middle Eocene or younger.
ROCKS OF THE CENOZOIC CASCADE MAGMATIC ARC
The oldest known Cascade arc rocks in the Mount Baker quadrangle are the 34-Ma gabbro of Copper Lake and the 32-Ma granodiorite of Mount Despair, early phases of the Chilliwack composite batholith. The birth of the Cascade magmatic was about 36 Ma (Vance and others, 1987; Smith, 1993). Arc-root plutons of the batholith range from gabbro to alaskite in composition and from 32 to 2.5 Ma (Oligocene to Pliocene) in age. In the quadrangle, plutons of the batholith with ages of about 30 Ma and older appear to belong to the Index family of arc-root plutons as defined by Tabor and others (1989). Those in the range of about 30 to 20 Ma are in the Snoqualmie family, and those younger than 20 Ma are in the Cascade Pass family.
Volcanic rocks of the Cascade magmatic arc are sparsely preserved in scattered down-faulted blocks or caldera-fill deposits. They commonly were erupted on eroded early phases of the Chilliwack composite batholith and then intruded by younger phases. The volcanic rocks of Big Bosom Buttes, of Mount Rahm, and of Pioneer Ridge range from dacite to less common andesite and basalt in composition and are probably Oligocene in age. The Hannegan Volcanics are mostly rhyolitic to dacitic and erupted in the Pliocene. The volcanic deposits of Kulshan Caldera are mostly rhyodacites and are Pleistocene. The Kulshan deposits underlie andesitic breccia and lava of the Mount Baker volcanic center, the youngest part of which includes Mount Baker itself, an active calc-alkaline stratovolcano.
QUATERNARY GLACIAL AND NON-GLACIAL DEPOSITS
Glaciations in the Mount Baker Quadrangle are recorded by deposits of both alpine and ice-sheet glaciers. Valley-bottom and valley-wall deposits in the upland trunk drainages (such as Big and Little Beaver Creeks, Silver Creek, Perry Creek, Goodell Creek, Thunder Creek, and the Cascade River 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 of Armstrong and others (1965), about 20,000 yr B.P., but were probably augmented during the Vashon stade, about 15,000 yr BP, when the high peaks in the eastern two-thirds of the Mount Baker quadrangle appear to have once again been a significant ice source. Additional deposits have been derived from lesser expansions of these same glaciers in Holocene time.
In the western part 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 of these deposits date from the Vashon stade of the Fraser glaciation culminating about 15,000 yr BP (Booth, 1987). Unvegetated moraines and outwash are common in many alpine cirques in the quadrangle, especially below still-active alpine glaciers.
Landslides, many of them still active, ornament slopes throughout the quadrangle. Large, probably catastrophic, slides came down into the valley of the North Fork of the Nooksack River, the Skagit River valley, and the Baker River valley. The Baker River slide is probably latest Pleistocene in age. The North Fork Nooksack and Skagit slides are clearly Holocene.
Generalized geologic map of the Mount Baker Quadrangle.
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