NEOGENE TECTONIC AND CLIMATIC RECORDS IN THE MARINE SEDIMENTARY STRATA OF THE ST. ELIAS MOUNTAINS AND THE GULF OF ALASKA

Abstract:

Over 5 km of Neogene marine sedimentary strata are well exposed in the St. Elias Range along the coast of Alaska. Immediately offshore in the Gulf of Alaska, the cores from the IODP Expedition 341 document correlative strata but provide a better temporally constrained record of more distal marine depositional processes. Together the onshore and offshore sedimentary records contain a transition from proximal to distal marine environments and provide insight on tectonics and surface processes interactions across a glaciated convergent margin. In this ongoing study, we document changing tectonic and climatic signals using U-Pb detrital zircon geochronology, zircon fission track thermochronology, and sedimentary lithofacies. For example, upper Miocene proximal strata consist of mudstone and sandstone that contain hummocky stratification that we interpret as being deposited in marine shelf environments. U-Pb detrital zircon ages from this unit have dominant populations from 59-63 and 85-90 Ma. In the distal submarine fan record, the upper Miocene strata consist of bioturbated mud and coarse sand that we interpret as marine sedimentary gravity flows followed by longer periods of hemipelagic deposition. The detrital zircon ages from these strata have dominant age populations from 53-54 and 60-80 Ma. The similar detrital ages from both the upper Miocene proximal and distal environments requires a common sediment link across the entire convergent margin by this time and the start of tectonic uplift of the modern mountain range. The start of Northern Hemisphere glaciation at 5.5 Ma in the proximal record is marked by submarine gravity flows interpreted to be reworked from alpine glacial deposits but to date there is no clear evidence of this climatic event in the distal record. The first clear record of glacial sediment input in the distal submarine fan environment is during the Plio-Pleistocene transition and recorded by ice-rafted muddy diamict beds.