AN ATTEMPT AT MASS BALANCING VOLUME OF EXHUMED ROCK FROM THE CENTRAL AND WESTERN ALASKA RANGE WITH THE VOLUME OF PLIOCENE AND QUATERNARY SEDIMENTS DEPOSITED IN BASINS OF SOUTH-CENTRAL ALASKA

The late Tertiary Sterling Formation in the Cook Inlet and Susitna Basins was eroded from the central and western Alaska Range. Apatite fission track and ⁴⁰Ar/³⁹Ar dating show that the Sterling Formation's deposition (≤ 4.6 to 9.4 Ma) coincides with rapid uplift of Mt. McKinley (Denali) (~6 Ma) and the western Alaska Range (≤ 5-10 Ma). Assuming no Pliocene and Quaternary sediment leaves Cook Inlet and no sediment flux from the Chugach Mountains, a mass balance of exhumed and deposited sediment was attempted. The volume of the Pliocene Sterling Formation and overlying Quaternary sediments was estimated from its outcrop extents, a published isopach map and gravity data. For estimating the volume of exhumed sediments, a smoothed surface over modern topography was digitally constructed. The central and western Alaska Range was then divided into four regions in which the amount of exhumation was considered separately. The amount of exhumed material was constrained by apatite fission track data for two of the regions, and by assumptions derived from geologic mapping where data was not available. Based on the smoothed surface, a theoretical higher surface representing the volume of exhumed sediments was constructed using an exponential relationship between the lowest elevation in a region and the highest elevation in a region plus the thickness of the exhumed rock column. Subtracting this theoretical higher surface from the present day surface yields the volume of rock eroded. A comparison between exhumed and deposited sediment volumes shows a 44% deficit of deposited sediments, in the simplest scenario. Reasonable modifications to the geotherm and eroded Sterling Formation volume can reduce this number to about 25%, a remarkably small amount considering various broad assumptions used. The remaining differences highlight the lack of information about assumptions and data used, including the amount and timing of exhumation of mountains in southcentral Alaska, the extent and volume of eroded Sterling Formation, the geothermal gradient, and sediment transport out of Cook Inlet. In particular, the wash load, that is the permanently suspended sediment load, can make up all of the volume deficit remaining, and may result in a sediment surplus. If so, then a more realistic model including some sediment contribution from the Kenai and Chugach Mountains can be included.