Principal Features. The peninsula known as Davidson Head, located at the northern extremity of San Juan Island, is composed of conglomerate, shale, slate, sandstone, graywacke, grit, and limestone of upper Triassic age. These rocks, which occupy an area of only 48 acres, make up what is here called the Haro formation. So far as it is definitely known, no other rock outcrops of this age occur in the San Juan Island region.
Lithology and Structure. The lowermost strata exposed on Davidson Head are made up chiefly of conglomerate with occasional thin interbeds of reddish colored sandstone and shale. The sandstone layers pinch out along the strike and conglomerates take their place. The shales frequently show spheroidal weathering. The conglomerates generally have a coarse greenish-colored matrix, although in places the matrix is calcareous and stained with ferric oxide. In many cases the boulders of the conglomerate are not well sorted, for large subangular fragments occur along with smaller pebbles of various sizes. The boulders are composed of fragments of the Orcas chert and Leech River graywacke, granite, dacite porphyry with virtreous phenocrysts of sanidine up to half an inch in diameter, dark greenish altered andesite, and fine-grained bluish-gray limestone. The thickness of the conglomerate member as exposed on Davidson Head is 920 feet.
The conglomerates are overlain by thin-bedded carbonaceous shale, slate, graywacke, grit, and limestone. The layers of limestone are interbedded with carbonaceous shale and the individual strata do not exceed four feet in thickness. The uppermost strata of the Haro formation are largely concealed by glacial drift, but the location of the fault line between the rocks of the Haro formation and the Orcas cherts is evident from the surface features.
The rocks occurring on Davidson Head are somewhat shattered and dislocated so that the dip and strike vary considerably from place to place. However, the average strike is nearly east and west and the dip is moderately steep towards the south. The sediments belonging to the Haro formation have not been intruded by igneous rocks within the region in which these rocks are exposed. The total thickness of the Haro formation occurring in the vicinity of Davidson Head is 1250 feet. Aside from a moderate amount of induration these rocks have suffered from a relatively slight amount of metamorphism.
Age and Correlation. Nearly all of the strata which overlie the conglomerates are abundantly fossiliferous, but the fossils occur only as impressions. All of the fossils collected belong to the genus Halobia, which is restricted in its occurrence to the upper Triassic.
The limestones are metamorphosed to such a degree that only the distorted outlines of Halobia can be distinguished. The fossils are best preserved in the carbonaceous shales and slates which occur just below the limestones. In many cases the shells have been replaced by pyrite and marcasite.
The conglomerates occurring on Upright Head, Humphreys Head, north end of Blakeley Island, Decatur Island, James Island, southeast portion of Orcas Island, Obstruction Island, Peapod Rocks, and Sinclair Island, all of which being located in the upper part of the Leech River group, contain dacite porphyry and andesites along with fragments of chert and graywacke. On first examination the writer considered these rocks to be Jurassic or even of later age, because it was supposed that the andesites and dacite porphyry belonged to the Vancouver volcanics. The conglomerates of the Haro formation also contain similar andesites and dacite porphyry. Dikes of similar material actually cut the Orcas cherts on Turtleback Mountain Range, but their relationship to the other igneous intrusions could not be determined. Although the rocks at the above mentioned localities have been considered with the Leech River group, it is possible that they should be correlated with the Haro formation.
Last Updated: 28-Mar-2006