CORRELATING THE ENDICOTT MOUNTAINS ALLOCHTHON, THE AMAWK THRUST, AND THE REGIONAL PRE-MISSSISSIPPIAN UNCONFORMITY ACROSS THE MT. DOONERAK ANTIFORM INTO THE CENTRAL BELT, CENTRAL BROOKS RANGE, ALASKA (Invited Presentation)

The Amawk thrust and the underlying structural window in the Mt. Doonerak antiform in the central Brooks Range were originally recognized by Gil Mull, who estimated over 90 km of northward displacement on the thrust. The parautochthonous lower plate consists of deformed pre-Mississippian metavolcanic and metasedimentary rocks overlain on a regional unconformity by the Mississippian to Triassic Ellesmerian Sequence, units that extend northward across the North Slope. Upper plate rocks on the northern flank consist of latest Devonian to Jurassic strata of the north-vergent Endicott Mountains allochthon (EMA). The southern flank consists of a low-grade metamorphic assemblage of Devonian and older strata assigned to the central belt (CB) of the Brooks Range because of their metamorphic character. Only the Frasnian (lower Upper Devonian) Hunt Fork Shale (Dhf) is recognized on both flanks of the antiform, with it being the basal unit of the EMA and the youngest unit in the CB. On the southern flank, the lowest Dhf is marked by local units of quartz-rich sandstone, sometimes overlain by thin fossiliferous units of Frasnian limestone, and locally intruded by mafic sills and dikes having rift-like geochemistry. The sandstone rests directly on various lithologic units, including Middle Cambrian metaclastic rocks, Silurian-Middle Devonian platform carbonate, and Middle Devonian felsic metavolcanic and metagraywacke rocks, which together provide evidence of an angular unconformity at the base of the Dhf. Units below the unconformity have significant DZ peaks at 600-650 Ma indicative of Timanian sources, whereas the Dhf quartzite above lacks those peaks and has peaks at ~400 Ma, similar to the Dhf in the EMA.

We conclude a major sub-Frasnian unconformity exists in the CB that separates post-rift fill (Dhf) from deformed Middle Devonian rift and older pre-rift units. With the Amawk thrust restored, this unconformity appears to young progressively northward from the CB to the sub-Mississippian unconformity in the core of the antiform and north to the North Slope. In contrast, the Amawk thrust must ramp down to the south under the south-flank stratigraphy in a hanging wall cut-off relation, suggesting the CB comprises the stratigraphically lower part of the EMA. Folding of the Mt. Doonerak antiform occurred later in Cenozoic time.