A Possible Reincised Eocene Paleovalley and New Formation near Brooks Camp, Naknek Lake, Alaska

Previously unmapped sedimentary rocks have been identified northeast of Brooks Camp, Naknek Lake, AK. They comprise sandstone and pebble conglomerate, with rare, thin silty sandstone beds and horizons exhibiting slight pedogenic alteration, and contain poorly preserved leaves. The leaves are probably late middle Eocene in age, consistent with minimum ages provided by diorite dikes that penetrate the section. The contacts of the sedimentary rocks with the underlying rocks are not exposed, but from general field relations and topographic considerations, they are almost certainly in angular unconformable contact with the regional Jurassic sedimentary rocks. We interpret the Eocene rocks to be a remnant of valley fill that was deposited in a paleovalley incised into the Jurassic rocks that was subsequently reincised. Although there are too few well-preserved angiopserm leaf morphotypes to definitively determine the climate, they are generally consistent with a community dominated by angiosperms in a warm temperate climate. Fossil wood suggests that the canopy was coniferous, and the growth rings in the wood are also consistent with such a climate. A few small outcrops of unnamed Tertiary sedimentary rocks are scattered on the NW side of the Alaska Peninsular Range. These outcrops are poorly described and poorly known. Published descriptions suggest a possible relationship with the rocks near Brooks Camp. In contrast, the rocks near Brooks Camp do not closely resemble the rocks described from the southeast side of the peninsula; paleocurrent directions are to the N; and onset of the arc edifice is coeval with or predates the deposition of the rocks. We therefore tentatively propose a type section and new name, Kittiwick Formation (for the previous name of Brooks Falls), pending further work on the nearby scattered outcrops of Tertiary rocks in the region.