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SUMMARY OF THE GEOLOGY OF THE MOUNT BAKER 30-MINUTE BY 60-MINUTE QUADRANGLE, WASHINGTON

Rocks in the Mount Baker quadrangle represent almost all the geologic events recorded in the entire North Cascades: (1) pre-mid-Cretaceous assembly of Mesozoic and Paleozoic terranes that have different paleogeographic origins and structural and metamorphic histories (Tabor and others, 1989; Tabor 1994), (2) mid- to Late Cretaceous thickening by thrusting and pluton accumulation (Misch, 1966; McGroder, 1991; Brown and Walker, 1993; Haugerud and others, 1994), accompanied and followed by regional metamorphism, (3) Eocene strike-slip faulting, extensional faulting, basin development, and continued metamorphism and plutonism (Johnson, 1984, 1985; Brown, 1987; Miller and Bowring, 1990; Haugerud and others, 1991; Miller, 1994), (4) growth of the Cascade magmatic arc in Oligocene to Holocene time (Vance and others, 1986; 1987; Smith, 1993; Tabor and others, 1989), and (5) Quaternary glacial erosion, drainage derangement, and deposition of glacial-derived sediments (Booth, 1987, 1990).

We summarize this geology here. More detailed discussions of bedrock lithologies, young volcanic rocks of Kulshan Caldera and Mount Baker volcanic center, unconsolidated deposits and Quaternary history, complete with more detailed references, are in the text accompanying I Map 2660.

Fig. 1. Crystalline core of the North Cascades, flanked by little metamorphosed rocks.

The Straight Creek Fault and the Ross Lake Fault Zone divide the rocks of the quadrangle into a core of deep-seated, thoroughly metamorphosed rocks, flanked by less metamorphosed rocks on either side (figs. 1, 2). These major faults are thought to be predominantly strike-slip (Misch, 1977a; Vance and Miller, 1981, 1992; Miller, 1994), though the rocks of the metamorphic core have been uplifted 15 to 25 Km relative to rocks on either side. The Straight Creek Fault, although now predominantly obliterated by Tertiary arc plutons, almost bisects the quadrangle. It separates core rocks from the Northwest Cascades System on the west. Estimates of right-lateral strike slip on the Straight Creek Fault range from about 90 to 190 km (Vance and Miller, 1981, 1992; Vance, 1985; Monger in Price and others, 1985; Kleinspehn, 1985; Coleman and Parrish, 1991; McGroder, 1991). Miller and Bowring (1990) described structural evidence of an early episode of strike slip on the Ross Lake Fault, and Haugerud (1985) and Miller and others (1994) indicated a strong discontinuity in metamorphic grade and history across the zone. Kriens (1988) and Kriens and Wernicke (1990a, b) suggested that the Ross Lake Fault Zone is a minor dislocation in an essentially uninterrupted cross-section of a Mesozoic arc ranging from the deep roots in the North Cascade core to the unmetamorphosed marine and terrestrial fore-arc deposits of the Methow terrane to the east.

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Generalized geologic map of the Mount Baker Quadrangle.
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