The Grandy Ridge-Lake Shannon area contains four major lithologic units: the Chilliwack Group, the Yellow Aster Complex, and the informally named chert/basalt and Triassic dacite units. The units are juxtaposed along anastomosing low angle faults of Late Cretaceous age. Additional deformation took place at a more recent time.
Lithologies of the Chilliwack Group predominate in the study area, with fine-grained sedimentary rocks of the lower clastic sequence present at lower elevations in the map area, and relatively mafic volcanic rocks present mostly at higher elevations. Sedimentary rocks in the vicinity of Upper Baker Dam, originally mapped as part of the Nooksack Group, are in this study assigned to the Chilliwack Group, based on lithologic, metamorphic, and structural considerations. The Chilliwack Group contains metamorphic mineral assemblages indicative of high pressure-low temperature metamorphic conditions. Reibeckite and crossite are reported for the first time in this unit.
Lithologies of the Chilliwack Group are present at the structurally lowest levels in the map area. A low angle thrust contact separates these rocks from overlying rocks of the Triassic dacite unit in many locations. The chert/basalt unit appears to be the structurally highest unit in the study area.
Evidence of two deformations is present in the Chilliwack Group. An early (D1) deformation is manifested by a persistent, low angle, slaty to phyllitic cleavage (S1) in fine-grained rocks, a northwest-trending stretching lineation (L1) in volcanic and coarse clastic rocks, and by infrequent northeast-trending folds. The second deformation (D2) is less extensive, primarily manifested by northwest-trending F2 folds.
The L1 lineations consist of stretched clasts and amygdules, and are most common along the top of Grandy ridge and in the vicinity of Upper Baker Dam. They are interpreted to represent the direction of shearing during the first deformation. Study of shear sense indicators suggests that the upper plate moved northwest relative to the lower plate. Strain magnitudes associated with these L1 lineations vary, but average approximately 3.5:1 in the XZ principal plane. This evidence suggests a minimum of several kilometers of northwest displacement of the approximately one kilometer thick section of rock exposed in the study area. The first deformation appears to have post-dated crystallization of the high pressure minerals, as evidenced by the presense of cracked and boudinaged lawsonite grains.
Evidence for northwest-southeast directed movement is present elsewhere in the Chilliwack Group, and is also present along segments of the Shuksan Fault. This movement may be related to emplacement of the structural units present in the western North Cascades
