Wrangell - St Elias National Park and Preserve Geology

LANDSCAPE

Wrangell-St. Elias National Park and Preserve is located in southcentral and southeastern Alaska and is an integral part of the greater Copper River ecosystem. The landscape is dominated by the eastern Alaska, Wrangell, St. Elias and Chugach Mountain ranges. Several of North America's highest peaks are within the park including Mt. St. Elias (18,008 feet) and Mount Wrangell (14,005 feet), an active volcano. From these mountains flow hundreds of glaciers varying tremendously in size. The Malaspina is one of the largest piedmont lobe glaciers and the Nabesna Glacier is one of the longest valley glaciers. Other glaciers terminate at tidewater and are known for their surging and retreating. Extensive ice fields also occur within the mountain ranges. The greater Copper River Basin ecosystem lies in one of the most tectonically active regions in the world. Volcanic action, earthquakes and ice have shaped the landscape over the past millennia. The complex topography resulting from these actions has profoundly influenced habitat diversity, distribution of species, biotic processes, and climate.

DIVERSITY

The park is noted for its geological diversity. The Wrangell region has attracted researchers to investigate volcanism, glaciation, plate tectonics and quaternary geology. The Nizina and Chitistone Canyons are areas where the geologic record is well represented and extensively exposed. The geologic history clearly exhibits the dynamic nature of the processes involved in the formation of the Wrangell and St. Elias mountain ranges. Southern Alaska, is an area composed of allocthonous tectonostratigraphic terraces. Segments of five tectonostratigraphic terraces occur within the park boundaries. The terraces reflect episodes of rifting, basin development, formation of island and magmatic arcs, and orogeny. From north to south they are: Yukon-Tanana, Wrangellia, Alexander, Chugach and Yakutat. Accretion of these terraces (northward migration) during the late Mesozoic and Cenozoic produced this regions complex geologic history. Each terrane contains a different stratigraphic record and is separated by a fault from the adjacent terraces. Unraveling the deformational history resulting from crustal plate interactions and conducting geochemical analysis are the major focus of current geologic research.

Within the northeastern part the park and north of the Denali Fault, a small portion of the Yukon-Tanana terrane is located. This terrane consists predominately of highly deformed and metamorphosed Paleozoic sediments and volcanics. Cretaceous aged plutons intrude the metamorphics. Wrangellia lies to the south of Yukon-Tanana. It outcrops over the greatest extent within the park and forms the basement of the Wrangell-Volcanic Field. An assemblage of island-arc volcanic rocks (late Paleozoic age - Tetelna Volcanics) lies at the base of the Wrangellia terrane. This volcanic unit is overlain by limestone, shale, and chert sedimentary units of Permian age. The Nicolai Greenstone, a Triassic sequence of metamorphosed basalts flows rests unconformably upon the Paleozoic section. The Upper Triassic through Lower Jurassic stratigraphy consists of limestone, impure limestone, shale and chert recording a deepening sedimentary basin. Cretaceous aged clastic units overlie these units unconformably. Wrangell Volcanics, Tertiary (Miocene) and Quaternary in age, cap the section.

Wrangellia is thought to have originated at lower latitudes prior to being transported northward and joined to mainland Alaska. A sliver of Late Jurassic-Early Cretaceous flysch (Nutzotin Sequence) and a remnant of the Cretaceous Chisana volcanic arc lie between the Totschunda and Denali Faults. Along the south side of the Wrangell Mountains metamorphosed equivalents of the Tetelna Formation, Jurassic Chitina Valley batholith and Late Paleozoic gabbros crop out. The southern boundary of Wrangellia is the Border Ranges Fault. A small part of the Alexander terrane, consisting of Paleozoic marbles, crop outs proximal to the international boundary. This terrane had amalgamated with Wrangellia prior to the Mid- Cretaceous and accretion along the coast of Alaska. The Chugach terrane is a belt of Lower Jurassic to Upper Cretaceous flysch (graywackes and slates) which has been intensely folded and weakly metamorphosed. It parallels the Gulf of Alaska along the coastal regions of Southern Alaska. Together with the Prince William Sound terrane (subterrane of the Chugach Terrane), it records intermittent accretion of volcaniclastic and volcanic (basalt) rocks from Early Jurassic to Eocene.

The Yakutat terrane, is an actively accreting terrane situated on an oceanic crust basement. The Contact Fault lies along its' northern border with the Chugach Terrane/Prince William Sound Terrane. It contains Eocene and younger sedimentary rocks (graywacke and shale) with minor basalt. The terrane traveled northward along the Fairweather Fault system prior to docking along the Gulf of Alaska. Recent Geology: Wrangell Volcanic Field covers more than 400 square miles and extends from the international border to Glennallen. The field is a continental volcanic arc which developed along a convergent plate margin above a Yakutat block related subduction zone. Heat generated that subduction zone generated the volcanic magma source. Quaternary surficial deposits are found within the park. The most extensive of these are glacial, lake and fluvial. Other Quaternary deposits include beach and shallow marine sediments along the coast, eolian deposits, and mass wasting (rock glaciers, debris flows and slides and solifluction) features. The area is seismically active because the Yakutat terrane is accreting to North America. The associated volcanism and active faults zones generate frequent earthquakes.