HISTORY OF BROOKS RANGE EXPLORATION AND DISCOVERY: A PERSONAL PERSPECTIVE

MULL, C.G., P.O. Box 117, Santa Fe, NM 87504, [email protected]

Geological exploration of the central and western Brooks Range began in 1901 and was exclusively by the U.S. Geological Survey (USGS) until the late 1950's. In the northeastern Brooks Range the results of privately funded exploration by Ernest Leffingwell were published by the USGS in 1919. In the mid-1920's and mid-1940's to 1950's, the USGS conducted extensive mapping and stratigraphic studies throughout the North Slope and into the mountains. These pioneering geologists mapped the bedrock distribution of large portions of the Brooks Range and North Slope, established the stratigraphic framework, and began to understand the structural style. They recognized the asymmetric Colville foreland basin north of the mountains, the contrast in stratigraphy and structural styles of the northeastern and central Brooks Range, the crustal shortening in the frontal portion of the central and western parts of the mountain belt, and thrust juxtaposition of contrasting far-travelled stratigraphic packages now termed allochthons. Beginning in the late 1950's and into the 1960's and 1970's, oil industry geologists began regional mapping, building on the USGS framework, filling gaps in the map coverage, and interpreting or reinterpreting the structural style and tectonic evolution of the range. Significant geological discoveries during this period included the Lower Cretaceous unconformity (LCU) in the northeastern Brooks Range, olistostromes and ophiolites in the western Brooks Range, and the Doonerak fenster in the central Brooks Range, but because of industry proprietary concerns, most of the new discoveries were published by the USGS. The recognition of the Doonerak fenster deep within the range showed that the frontal parts of the central and western Brooks Range are allochthonous on a scale of many tens of miles and contrast with parautochthonous rocks of the northeastern Brooks Range. The advent of plate tectonic concepts in the late 1960's resulted in interpretation of the Brooks Range as an alpine orogenic belt formed by Jurassic to Cretaceous obduction of a Pacific oceanic arc over North American continental crust of the Arctic Alaska microplate, with several hundred miles of crustal shortening followed by mid- to Late Cretaceous rotation of Arctic Alaska away from the Canadian Arctic Islands.