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The geology of Lummi and Eliza islands, Whatcom County, Washington Calkin, Parker Emerson

Abstract

Lummi and Eliza Islands form the northeast part of the San Juan Island group in northwest Washington. Lummi is a long, narrow island characterized by a rocky, mountainous southern half and a low, northern half. Eliza is a small T-shaped island southeast of Lummi Island. Lummi Island is underlain by igneous, metamorphic, and sedimentary rocks of Paleozoic to Lower Cenozoic age. The oldest rocks are believed to be those of the Lummi Island Metamorphic and Igneous Complex which form a small, isolated knob in the middle of the island. These are hornblendic rocks, intruded by quartz-albite rocks and cut by numerous aplite and lamprophyric dikes. The age and origin of these rocks is unknown but they may be older "basement" rocks brought to their present position through faulting. Shale, graywacke and granule conglomerate of the Carter Point formation (Paleozoic or Mesozoic) underly most of southern Lummi Island. These rocks show all the characteristics of the typical "graywacke suite" such as great thickness, clastic character, rhythmic bedding, and graded bedding. The only fossils found were a few carbonized plant stems imbedded in fine-grained graywacke. The rocks forming the bedrock of Eliza Island may be a more metamorphosed equivalent of these. Overlying the Carter Point formation on the southeast side of Lummi Island and directly underlying the sandstone at the northern end are the Reil Harbor volcanics. Although they occur in five isolated outcrops these rocks are grouped together on the basis of lithology and outcrop features. In contrast to an earlier intrusive interpretation these occur as submarine (pillow) lavas and interbedded breccia with tuffaceous - argillaceous rocks rather than as dikes or sills. The lavas of some of the outcrops are spilitic and in most cases are extremely altered. The breccias are dominantly volcanic - clastic types which show some reworking. The age of the volcanics and underlying Carter Point formation is unknown; however, interbedded sedimentary rocks contain radiolarian tests suggestive of Mesozoic age. Northern Lummi Island is underlain by plant-bearing lithic-feldspathic arenites and conglomerates of the Chuckanut formation (Paleocene). These are believed to have a continental fluviatile origin on the basis of: absence of marine fossils; conspicuous amounts of hematite imbedded in the sandstone; moderate sorting and rounding; apparent large-scale heterogeneity evidenced by internal structures such as prominant cross bedding and cut - fill structures, and the dominance of sandstone and conglomerate facies. The Carter Point formation and the overlying volcanics on the southeast side of Lummi Island strike N 40 W and dip 45 degrees NW. Drag folds suggest that southern Lummi Island represents the eastern limb of a northwest plunging anticline. The Chuckanut formation and the underlying Reil Harbor vol-canics at the north end of the island have been folded into three synclines which strike northwest-southeast and plunge gently northwest. During the Pleistocene, northern Lummi Island was blanketed with glacial drift while the higher knobs here and the rocks of southern Lummi were grooved, polished or eroded by the glaciers.

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