37 Decadal Symposium on the Geology of Washington I: In Honor of Rowland W. Tabor

Wednesday, 15 May 2002
8:00 AM - 12:00 PM

STRATIGRAPHIC RECORD OF CRETACEOUS TECTONICS IN THE METHOW BLOCK, NORTH CASCADES, WASHINGTON

Paper number: 37-5
Program: THEME SESSIONS
Time: 8:50 AM – 9:05 AM

Authors

Abstract

We divide strata of the Methow block south of 49ยบ 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.