Event Name : Togiak volcanics 760000 yBP
|Start: 760000 (± 200000 Years) || Years BP K-Ar || |Description:
|Eruption Product: || basalt ||
From Hoare and Coonrad (1980): "The Togiak Basalt consists largely of undeformed subaerial tholeiite and alkali-olivine basalt flows (table 1 [in original text]) characterized by well-developed columnar jointing and generally 3 m to 10 m thick with vesicular ropy tops. The columns are generally 1 to 2 m in diameter but are smaller in the finer grained flows. Most of the flows are gray, medium-grained, nonporphyritic rock with a loose diktytaxitic texture; some flows are dense fine-grained black rock with small olivine phenocrysts. The flows consist of labradorite, augite, olivine, and accessory magnetite. The fine-grained darker flows also have interstitial glass."
From Hoare and others (1978): "The widespread flows on the valley floor are preglacial in age. They apparently erupted from vents that were localized along the faults. Most of the older volcanic vents were probably destroyed by glacial ice, and only one of them has been identified. It is located about 11 km northeast of the tuya on a small shelf that projects out from the valley wall about 170 m above the valley floor (fig. 2 [in original text]). The shelf is underlain by highly oxidized, vesicular breccia, the probable remnant of an old preglacial cone. The location and elevation of the cone remnant suggest vertical uplift on the Togiak fault. A second, larger shelf at about the same elevation projects out from the valley wall east of the tuya. This shelf is underlain by horizontal flows. The flows and shelf can be traced up a tributary valley where the flows overlie highly deformed rocks of Mesozoic (Jurassic?) age. We conclude that there has been about 170 m. of vertical movement on the Togiak fault since the flows erupted on the valley floor."
"The radiometric age of a flow on the valley floor 20 km north of the tuya was determined by the potassium- argon method to be 0.76+/-0.2 million years (J. G. Smith, written commun., 1976)."
"We determined the magnetic polarity of the flows on the valley floor and on the tuya at several places and found that all of the flows are normally magnetized. The polarity of the flows and the radiometric age determination indicate that the flows on the valley floor erupted near the beginning of the present (Brunhes) polarity epoch, which began approximately 700 000 years ago (Cox and others, 1963)."
"The tuya is clearly younger than the flows on the valley floor because it overlies them."