EVIDENCE AND IMPLICATIONS FOR OUT-OF-SEQUENCE STRUCTURES IN THE SADLEROCHIT MOUNTAINS REGION OF THE ARCTIC NATIONAL WILDLIFE REFUGE, ALASKA

Fission-track thermochronology and structural analysis constrain the timing and structural development of the Sadlerochit Mountains, located along the southern edge of the coastal plain in the Arctic National Wildlife Refuge (ANWR) of northeastern Alaska. Rocks exposed south of the Sadlerochit Mountains within Ignek Valley experienced two episodes of rapid cooling from elevated paleotemperatures at ~45 Ma and at some time since ~31 Ma, whereas similar-aged rocks exposed along the northern flank of the Sadlerochit Mountains cooled rapidly at ~45 Ma and ~27 Ma. Additional results from the Beli Unit #1 well, located northwest of the Sadlerochit Mountains, indicate that the sampled sequence experienced progressive heating during the Late Cretaceous to Early Cenozoic prior to cooling rapidly at ~45 Ma. Combined, the thermochronology results indicate that the Sadlerochit Mountains region experienced progressive heating up to Middle Eocene time, after which two major episodes of rapid cooling occurred in the Middle Eocene at ~45±3 Ma and in the Late Oligocene, at ~27±2 Ma.

These episodes of rapid cooling are interpreted to have occurred in response to km-scale erosional denudation, which resulted from uplift due to structural thickening during the emplacement of horses in a basement-involved duplex. Initially, at least one horse was probably emplaced to the north of the Sadlerochit Mountains at ~45 Ma. Subsequently, at ~27 Ma: 1) the Sadlerochit Mountains horse was probably emplaced behind the earlier emplaced horse(s), and thus was out of sequence, and 2) basement-involved deformation formed structures beneath the coastal plain to the north.

These results indicate that potential hydrocarbon source rocks in the vicinity were exposed to peak burial temperatures prior to Middle Eocene time, after which they experienced rapid cooling during the initial episode of structural deformation. Therefore, hydrocarbon generation from these source rocks probably occurred before formation of the structures in and immediately north of the Sadlerochit Mountains.