Sandra H.B. Clark

Interpretation of a High-Grade Precambrian Terrane in Northern Idaho

A terrane of high-grade metamorphic rocks in northern Idaho and northeastern Washington is almost completely surrounded by low-grade rocks of the Precambrian Belt Supergroup. The high-grade terrane includes both Belt and pre-Belt rocks. Four events of folding and metamorphism occurred in the high-grade terrane. The first three events may have been associated with the Late Cretaceous emplacement of quartz monzonite of the Kaniksu batholith; the fourth may have been associated with a slightly later emplacement of granodiorite or with a Tertiary plutonic and volcanic episode. A much older event of plutonism in the high-grade terrane is recorded by zircon, which was dated by the Pb-U method at 1,500 m.y. from pre-Belt meta-igneous augen gneiss. Evidence of regional events intermediate in age between 1,500 and 100 m.y. has been found in the surrounding low-grade rocks but not in the high-grade terrane.

Growth of a Talus Cone in the Western Chugach Mountains, Alaska

A significant increase in the size of a talus cone near the terminus of Eklutna Glacier during the 6 yrs following the 1964 Alaska earthquake is shown by comparison of photographs taken in 1964 and 1970. Calculations from approximate measurements indicate that about 1.7 million cu yds of rock has been added to the cone since 1964. This growth has been the result of intermittent rockfalls which began after the 1964 earthquake and were continuing in 1970. Rockfalls in other places in the same area were initiated by the earthquake, but did not continue at a rate sufficient to produce measurable change in the talus. Observations on the ridge above the active talus cone suggest that the configuration of the cliff, fracturing and loosening of the rock at the time of the earthquake, and possible land movements resulted in a uniquely unstable area. The instability may be sufficient to account for the continuation of an unusually large number of rockfalls.