Lateral Variation in Devonian and Older Stratigraphy and Facies across the Sadlerochit and Shublik Mountains Region, Arctic National Wildlife Refuge, Alaska: Impact on Subsurface Coastal Plain Geology

By

J.G. Clough (Alaska Division of Geological & Geophysical Surveys)

Relationships of Precambrian through Lower Devonian strata observed in the Sadlerochit and Shublik mountains provide insight into pre-Mississippian petroleum play strategies north of the mountain front. Lateral facies changes combined with paleocurrent data across the region indicate that the Neoproterozoic-age Katakturuk Dolomite represents extensive carbonate ramp deposition terminated by a major karst event. Northeast-trending normal faults that are perpendicular to the prevailing platform margin facing direction (southeast) suggest these faults are extensional in nature and formed during Neoproterozoic rifting events associated with passive margin development. Abrupt lateral facies changes in the overlying Nanook Limestone (Cambrian, Ordovician) imply the northeast-trending faults were active in Early Paleozoic time as well, and that the Sadlerochit Mountains were a topographic high during most of Nanook deposition.

Erosion beneath a sub-Nanook unconformity has removed the entire Katakturuk section south of the Shublik Mountains. Pre- Mississippian strata in the Fourth Range contain deep-water trace fossils in slope to basin equivalents of the Nanook Limestone. Lower Devonian carbonate exposures, represented by the Mt. Copleston Limestone, are restricted to the Shublik Mountains and likely not deposited to the north, consistent with a topographic high, or removed by the interregional sub-Mississippian unconformity (sMu) that also removed significant portions of strata across the North Slope. Beneath the coastal plain north of the mountain front both the upper karst horizon of the Katakturuk Dolomite and the coated-grain facies throughout its entire section are an attractive petroleum reservoir target, where not removed by the sMu or the later Lower Cretaceous unconformity.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.