Thomas M. Parris¹, Robert C. Burruss²

(1) U.S. Geological Survey, Denver, CO
(2) U.S. Geological Survey, Reston, VA

ABSTRACT: Tectonism, Fracturing, and Gas Generation, Northeast Brooks Range, Alaska

Fractures cemented primarily with quartz in Triassic Ivishak and Shublik Formations and Jurassic Kingak Shale are exposed in a thrusted anticlinorium along the Aichilik River at Leffingwell Ridge. Petrographic and SEM-cathodoluminescence studies of the quartz cements reveal syntectonically bent crystals and cross-cutting fractures, with many fractures containing crack-seal textures.

Thermometric analysis of fluid inclusions in the quartz suggests crystal growth at 8 to 10 km (homogenization temperatures=205- 250°C), assuming a geothermal gradient of 25°C/km and surface temperature of 0°C. Gas-rich inclusions are CH4 and CO2 mixtures (XCH4=0.79- 0.9), which is consistent with gas generation from the Shublik and Kingak at high thermal maturity at the time of fracturing. Entrapment pressures range from 1000 to 2000 bars, with most estimates in the range of 1100 to 1600 bars.

Homogenization temperatures for the quartz cement overlap zircon fission-track closure temperatures (~240°C), which were used to date nearby uplift (~45 Ma) considered to represent a major period of folding and thrusting. The overlap suggests that the fractures developed during this period of northeast Brooks Range deformation and uplift. From 45 Ma, time-averaged uplift rates of ~0.2 mm/yr are needed to get the samples to the present-day surface, which is well within the range of transient periods of more rapid uplift (up to ~2 mm/yr) documented by fission track analyses.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado