Sedimentology and Stratigraphy of the Sag River Formation, Northern NPR – Alaska
Authors: Barnes David A. 
Published: November 2010

The Sag River Formation is a minor (0-40m thick) but economically significant lithostratigraphic unit in the upper Ellesmerian megasequence in NPR-Alaska. The Sag River can be readily correlated to strata found elsewhere in the Arctic North Slope Province. The Sag River interfingers with the upper Shublik Formation (Zone A) in the northern NPR-Alaska where localized, coarser-grained sandy clastic facies, depositional thinning and pinchout due to onlap, and possible disconformity onto the basin margin confound stratigraphic correlation. The Sag River and related strata in the NPR-Alaska are subdivided into heterolithic; sandstone; and argillaceous, silty sandstone facies. These facies occur in a generally coarsening upwards succession that thins and losses integrity from proximal to distal settings, generally from north to south.

The Sag River was deposited in south facing passive continental margin basin on a low relief but irregular (along strike) marine depositional surface with localized, exposed basement shoals and shallow bathymetric depressions. The depositional environments represented in the Sag River developed along a distal, lower shoreface to upper offshore depositional profile in an open marine shelf setting. Episodic storm events in an otherwise relatively low wave energy, paralic setting with relatively slow rates of fine-grained terrigenous clastic mud, silt, and sand deposition resulted in a highly bioturbated silty mudstone to argillaceous sandstone sedimentary facies assemblage. Slow sediment accumulation rates and distinctive chemical oceanographic conditions associated with probable elevated biological productivity resulted in the formation of variable amounts of syndepositional, peletal glauconite and replacive phosphate in the Sag River Formation.

The Sag River Formation is interpreted as a progradational, HST deposit which gradationally overlies generally clastics poor facies of the underlying Shublik Formation Zone A. Transition to retrogradational marine deposition in a TST at the top of sandy facies probably resulted from accelerated relative sea level rise during later Sag River time. The speculative MFS at the top of the Sag (which is almost certainly present in the Prudhoe Bay area) is overlain by progradational deltaic clastics of the Kingak Shale and related stratigraphic units at a regionally significant downlap surface, probably marking the onset of tectonic rejuvenation of the Barrovian hinterlands to the north in association with the onset of Beaufortian continental rift rifting.

The onset of predominantly clastic Sag River Formation sedimentation during regional progradation, in a HST which had been dominated by organic-rich and chemical sediments of the Shublik Zone A, may have been the result of climate change from more arid to more humid and greater rainfall conditions in latest Triassic time. Neither significant sea level fall nor nebulous hinterland tectonics seem compelling mechanisms for the increased rate of arenaceous clastics input during the Sag River regression. Climate change from arid conditions, possibly associated with coastal upwelling during Shublik time, to increased rainfall and increased fluvial discharge during Sag River Formation time may have resulted in the distinctive stratigraphic transition from Shublik to Sag River formation deposition throughout the Arctic North Slope Province.