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Out-of-sequence, basement-involved structures in the Sadlerochit Mountains region of the Arctic National Wildlife Refuge, Alaska: Evidence and implications from fission-track thermochronology

Paul B. O'Sullivan^,1 and Wesley K. Wallace^,2

¹ Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA
² Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775, USA

Fission-track thermochronology and structural analysis set limits on the timing and nature of structural development of the Sadlerochit Mountains, along the southern edge of the coastal plain in the Arctic National Wildlife Refuge (ANWR) of northeastern Alaska. The Sadlerochit Mountains are the northernmost part of the north-vergent Brooks Range fold-and-thrust belt and lie close to the Arctic continental margin. Thermochronology results indicate that sedimentary rocks exposed within Ignek Valley, south of the Sadlerochit Mountains, were subjected to two episodes of rapid cooling from elevated paleotemperatures at ca. 45 Ma and at some time since ca. 31 Ma, whereas similar-aged rocks exposed along the northern flank of the Sadlerochit Mountains cooled rapidly at ca. 45 Ma and ca. 27 Ma. Combined with five additional analyses from the Beli Unit #1 well, located northwest of the Sadlerochit Mountains, the thermochronology results indicate that the Sadlerochit Mountains region was progressively heated during Late Cretaceous through middle Eocene time, after which two major episodes of rapid cooling occurred in the middle Eocene at ca. 45 ± 3 Ma (±2) and in the late Oligocene at ca. 27 ± 2 Ma (±2).

These episodes of rapid cooling are interpreted to have occurred in response to kilometer-scale erosional denudation resulting from rapid (5 m.y.) uplift due to structural thickening during the emplacement of thrust sheets in a basement-involved duplex. Initially, at least one thrust sheet was probably emplaced to the north of the Sadlerochit Mountains at ca. 45 Ma. Subsequently, at ca. 27 Ma, (1) the Sadlerochit Mountains thrust sheet was probably emplaced out of sequence behind the earlier-emplaced thrust sheet(s), and (2) basement-involved deformation formed structures beneath the coastal plain to the north. Both of these events occurred far within the continent, >1200 km from the southern Alaska convergent plate boundary.

These results indicate that maximum burial, and hence peak hydrocarbon generation, occurred prior to middle Eocene time, before the formation of potential traps in and immediately north of the Sadlerochit Mountains. However, (1) hydrocarbons generated from these rocks could have migrated updip into existing stratigraphic traps prior to structural deformation, and (2) hydrocarbons generated later in more distal parts of the basin could have migrated updip into subsurface structures formed in middle Eocene and late Oligocene time north of the Sadlerochit Mountains and along strike to the east and west.

Key Words: Alaska • Arctic National Wildlife Refuge (ANWR) • basement-involved thrusting • Brooks Range • fission-track dating • out-of-sequence thrusting • Sadlerochit Mountains • thermotectonics

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