The area between Knik and Turnagain Arms east of Anchorage is underlain mostly by rocks that are part of an extensive arcuate belt of thick Mesozoic marine deposits that extend through the Chugach-Kenai-Kodiak Mountains. The two main units in this belt are the Jurassic (?) and Cretaceous Valdez (?) Group composed of flysch deposits and the McHugh Complex composed of oceanic metavolcanic sequences tectonically mixed with metaclastic rocks derived from a continental magmatic arc. Deformation of the McHugh Complex is characterized by pervasive, closely spaced shear fractures and is melange-like in some areas. Deformation of the Valdez (?) Group is characterized by tight folding that was initiated before the sediments were completely lithified. The folds have steeply dipping axial surfaces that are overturned to the northwest in some areas and to the southeast in other areas. The Jurassic and(or) Cretaceous McHugh Complex is separated from the Valdez (?) Group by the Eagle River thrust fault. The Valdez (?) Group was probably deposited primarily on oceanic crust that was collapsed against the continental margin in latest Cretaceous to early Tertiary time.
An accurate belt of upper Paleozoic to lower Mesozoic rocks to the north and west had been deformed, accreted to the continental margin, and intruded by plutons prior to the deposition of the Jurassic (?) and Cretaceous sediments. Rocks of the continental terrane that underlie the Wrangell and Talkeetna Mountains and the Alaska Range are locally exposed in the map area north and west of the Knik fault zone. Near the village of Eklutna, the junction between the two terranes is marked by an ophiolitic assemblage.
