GEOLOGIC MAP OF THE SNOQUALMIE 7.5-MINUTE QUADRANGLE, KING COUNTY, WASHINGTON—ACTIVE FAULTING, BASIN INVERSION AND MIOCENE VOLCANIC EXTRUSION OF THE SNOQUALMIE BATHOLITH ALONG THE RATTLESNAKE MOUNTAIN FAULT ZONE

A complex interplay between basin deposition, uplift, and intrusion within a broad active fault zone strike-slip fault zone were found in the Snoqualmie 7.5-minute quadrangle. Late Pleistocene to Holocene activity along strands of the Rattlesnake Mountain fault zone (RMFZ), a broad regional strike-slip fault zone, has created steeply-dipping faults, fractures and liquefaction features in both nonglacial Olympia beds and younger Vashon glacial deposits. Stratigraphic relationships and structural style suggest strike-slip deformational control of the basins since at least the early Miocene. Some Pleistocene deposits are distinctly deformed (folded and faulted). For example, sand provenance, radiocarbon ages, and structures show that the late Pleistocene nonglacial Olympia beds are ancient Snoqualmie River alluvium that has been (growth) folded, faulted, tilted and uplifted between strands of the RMFZ. The thickness and structural style of these beds suggest that they are entrained in RMFZ pull-apart basins and were locally uplifted to form shortened—formerly extensional—inverted basins. This basin inversion is hypothesized to be operative along the length of the RMFZ. New U-Pb age dates show that the informally-named volcanic rocks of Snoqualmie Falls (VRSF) are Miocene (~20 Ma)—a fault-dissected volcanic edifice or caldera centered near Snoqualmie Falls and preserved in the Snoqualmie basin. Proximal volcanism is indicated by the dominance of flows and bomb breccias (bombs to 9 ft long) and a lack of distal rocks. We suspect that the VRSF are an extrusive equivalent to the Snoqualmie batholith, based on their geochemical, petrologic, magnetic, and geochronological similarities. The batholith both underlies and is exposed in the map area along the RMFZ main strand. It seems likely that the batholith intruded along the thick, weak cataclasites of the RMFZ, extruding to form the andesitic rocks of the Miocene VRSF. The North Bend, Snoqualmie, and Snoqualmie Valley basins are other potential active basins within the RMFZ. Adjacent quadrangle mapping shows that: (1) the RMFZ probably merges with the Olympic-Wallowa lineament south of the quadrangle, (1) bounds the Seattle Fault zone northwest of the quadrangle and (3) correlates with the Southern Whidbey Island fault north-northwest of the map area.