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Facies Variation within Outcrops of the Triassic Shublik Formation, Northeastern Alaska

Professional Paper 1814-H
Studies by the U.S. Geological Survey in Alaska, Volume 15
By: , , , and 
https://doi.org/10.3133/pp1814H

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Abstract

The Shublik Formation (Middle to Upper Triassic) is a heterogeneous unit that is a major hydrocarbon source rock in northern Alaska and the largest known Triassic phosphate accumulation in the world. This formation, which occurs in the subsurface and crops out within the Arctic Alaska basin, was deposited on a gently sloping ramp along the northwestern Laurentian margin. In this study, we document spatial and temporal facies variations within the Shublik and the overlying Karen Creek Sandstone (Upper Triassic) through 19 outcrop localities in the northeastern Brooks Range. New organic and inorganic geochemical data from these localities support and extend our sedimentologic and petrographic findings. The petrographic data come from an analysis of more than 900 thin sections, whereas age constraints come mainly from species of the pelecypod genera Daonella, Halobia, Eomonotis, and Monotis. Thirteen sites make up a main outcrop belt within which facies of the Shublik are spatially consistent but show marked vertical changes. Six additional outcrops located south of the main belt contain more distal facies of the Shublik that accumulated farther from land.

Exposures of the Shublik Formation in its main outcrop belt are divisible into five informal lithologic units, each of which formed during a distinct transgressive to regressive sequence. The basal unit of the Shublik (Anisian? to Ladinian; Middle Triassic) is quartz siltstone to very fine grained sandstone that is locally phosphatic. The three middle units (Ladinian to middle Norian; Middle to Upper Triassic) contain various proportions of siliciclastic, carbonate, phosphatic, and organic material. The uppermost unit (middle to upper Norian; Upper Triassic) is mainly shale and mudstone. Highly phosphatic strata (10–34 percent phosphorus pentoxide) are chiefly Ladinian and lower to middle Norian. Highly phosphatic Ladinian strata have been transported, are granular, and formed during transgression and early regression. In contrast, highly phosphatic Norian strata include event beds and hardgrounds, contain displaced and in situ phosphate peloids and nodules, and cap regressive parasequences. The total organic content reaches 4.97 weight percent in the main belt and is highest in muddy beds, which are found mainly in the lower parts of the three middle units. Distal sections of the Shublik are typically finer grained, more organic-rich (total organic content as high as 6.33 weight percent), and less fossiliferous than those in the main belt.

Both temporal and spatial factors shaped facies in the Shublik Formation. Shublik strata accumulated at a time of reduced tectonic activity in Arctic Alaska, resulting in diminished siliciclastic input. Marine upwelling along the northwestern Laurentian margin facilitated the development of heterozoan carbonates, as well as local concentrations of phosphatic and organic matter. Large- and small-scale eustatic cycles also affected this margin and provided further controls on Shublik facies distribution.

Suggested Citation

Dumoulin, J.A., Whidden, K.J., Rouse, W.A., and DeVera, C., 2024, Facies variation within outcrops of the Triassic Shublik Formation, northeastern Alaska, in Dumoulin, J.A., ed., Studies by the U.S. Geological Survey in Alaska, v. 15: U.S. Geological Survey Professional Paper 1814-H, 49 p., https://doi.org/10.3133/pp1814H.

ISSN: 2330-7102 (online)

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Table of Contents

  • Abstract
  • Introduction
  • Geologic Setting
  • Materials, Methods, and Terminology
  • Stratigraphic and Structural Framework
  • Lower Clastic Unit 1
  • Middle Carbonate-Chert Unit 1
  • Middle Carbonate-Chert Unit 2
  • Middle Carbonate-Chert Unit 3
  • Upper Clastic-Carbonate Unit 1
  • Other Sections of the Shublik Formation
  • Karen Creek Sandstone
  • Facies Patterns
  • Conclusions
  • Acknowledgments
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Facies variation within outcrops of the Triassic Shublik Formation, northeastern Alaska
Series title Professional Paper
Series number 1814
Chapter H
DOI 10.3133/pp1814H
Year Published 2024
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Alaska Science Center Geology Minerals, Central Energy Resources Science Center, Eastern Energy Resources Science Center
Description Report: vii, 49 p.; Data Release
Country United States
State Alaska
Online Only (Y/N) Y
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