Geology
Henrietta Island consists of folded Middle Paleozoic basaltic lava and proximal volcanogenic turbidites overlain by Cenozoic clastic sedimentary rocks. The Paleozoic strata have been intruded by numerous sills, dikes, and sheets of basalts, andesite-basalts, and porphyritic diorite. The basalts and porphyritic diorite have been dated by potassium–argon dating method to be about 310-450 million years old and the porphyritic diorite has been dated by the argon–argon dating method to be about 400-440 million years old. Gritstones that are part of the Cenozoic clastic sedimentary rocks contain fragments of underlying Paleozoic strata along with significantly older gneisses, granites, quartzites, and schists.
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