| STRATIGRAPHIC RECORD OF CRETACEOUS TECTONICS IN THE METHOW BLOCK, NORTH CASCADES, WASHINGTON | ||
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HAUGERUD, Ralph A., U.S. Geol Survey, Dept. Earth & Space Sciences, University of Washington, Box 351310, Seattle, WA 98195, rhaugerud@usgs.gov, TABOR, Rowland W., U.S. Geol Survey MS 975, 345 Middlefield Road, Menlo Park, CA 94025, and MAHONEY, J. Brian, Univ Wisconsin - Eau Claire, 105 Garfield Ave, Eau Claire, WI 54701-4811 We divide strata of the Methow block south of 49º N into 5 Cretaceous-Tertiary sequences. (1) Fossiliferous, volcaniclastic, shallow marine to fluvial Lower Cretaceous strata include rocks at Dead Lake (Albian), the Patterson Lake unit (Albian), the type Panther Creek unit of Barksdale (1975) (Valanginian?), and sedimentary rocks on Buck Mtn (Barremian or older to Albian). We found no proximal volcanic rocks of this age. In the Lightning Creek drainage this unit is in part Jurassic. (2) Overlying Three Fools sequence comprises east-derived, quartzofeldspathic, mid-Albian, proximal turbidites of the Harts Pass Formation, overlain by--and inferred to interfinger with--the west-derived chert-clast-rich Jackita Ridge unit, also turbiditic but thinner bedded and siltier than the Harts Pass. Rare ammonites in the Harts Pass are mid-Albian; inoceramids from the Jackita Ridge are Albian to Cenomanian. (3) Pasayten Group overlies the Three Fools sequence, locally on an angular unconformity. Constituents of the Pasayten Group are west-derived, chert-clast-rich, and largely fluvial Virginian Ridge Fm; east-derived, arkosic, and largely fluvial Winthrop Fm; and redbeds and volcanic rocks of the Goat Wall unit. We abandon the Midnight Peak Fm of Barksdale (1975): volcanic rocks at its type locality are Three AM member of the Winthrop Fm; stratigraphically higher beds near Mazama are Goat Wall unit. (4) Campanian-Maastrichtian conglomerate of the Pipestone Canyon Formation is fluvial, largely proximal, and rich in rhyolite debris. It is overlain by undated basalt. (5) Isolated, mountain-capping, hornblende-phyric volcanic rocks (e.g. Storey Peak, Last Chance Point) may correlate with dated middle Eocene rocks farther east. Basin deepening at the transition between sequences 1 and 2 dates the onset of contraction in this part of the North Cascades as early to mid-Albian. Three Fools sequence (2) and Pasayten Group (3) are upwards-shallowing foreland-basin fill developed in front of, and cannibalized by, the east-vergent Hozameen-Chuwanteen thrust system. After Goat Wall time (~87 Ma) this region was within the Coast Belt hinterland. Pipestone Canyon Fm (4) records subsequent wrench faulting that in part defines the Methow block. | ||
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Cordilleran Section - 98th Annual Meeting (May 13–15, 2002)
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| Session No. 37 Decadal Symposium on the Geology of Washington I: In Honor of Rowland W. Tabor CH2M Hill Alumni Center: Ballroom 110B 8:00 AM-12:00 PM, Wednesday, May 15, 2002 | ||
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