Foreland Basin Response to Paleocene Rejuvenation in the Brooks Range, Northern Alaska

Marwan A. Wartes¹, Paul L. Decker², David W. Houseknecht³, Robert J. Gillis¹, and David L. LePain^1
1Alaska Division of Geological & Geophysical Surveys, Fairbanks, AK.
²Alaska Division of Oil & Gas, Anchorage, AK.
³U.S. Geological Survey, Reston, VA.

The Brooks Range and its coupled foreland basin record a major episode of Late Jurassic to mid-Cretaceous collisional orogenesis. However, geologic mapping of cross cutting structural relations and folded Late Cretaceous - Paleogene strata indicate younger deformation affected the region. Stratigraphy and thermochronologic data suggest most of this rejuvenation occurred in the Paleocene when much of northern Alaska underwent widespread uplift and denudation. Despite the significance of this exhumational event, little is known about its attendant stratigraphic record in the foreland basin.

Recent surface and subsurface stratigraphic studies on the eastern North Slope provide new insight on Paleocene depositional systems and sequence stratigraphy. At Sagwon Bluffs, near the Dalton Highway, a significant unconformity separates the Maastrichtian to Paleocene Prince Creek Formation from the Paleocene Sagwon member of the Sagavanirktok Formation. Abrupt changes across this intra-Paleocene surface include 1) increase in grain size, 2) sandstone and clast composition, and 3) reduced sinuosity of fluvial systems. These observations are interpreted as the record of uplift in the Brooks Range and increased gradient and sediment supply across the foreland.

Recent surface mapping farther east indicates this subaerial unconformity can be correlated basinward. Seismic and well data suggest the correlative Paleocene shelf margin was incised and deeply eroded during a major base level fall, resulting in a regional downward stepping trajectory of toplap surfaces. This surface can be traced into deep-water strata, where significant submarine scouring is evident. Wells drilled beyond this shelf margin typically exhibit a sharp dislocation in log motif, indicating an abrupt influx of sandstone in slope and basinal facies of the Canning Formation. Several oil-charged slope-channel and slope-apron turbidite systems have been discovered in association with this lowstand systems tract.

The greatest thickness of Paleocene strata is observed on the easternmost North Slope, reflecting the greater inherited accommodation in the remaining underfilled sector of the basin. However, this portion of the basin does not appear to record passive filling; instead, consideration of northward prograding and southward thickening tongues of Paleocene strata in the Canning River area are most consistent with synchronous tectonic subsidence in a flexural foredeep lying directly north and west of the growing orogenic wedge. This pattern of subsidence contrasts markedly with coeval uplift of much of the North Slope to the west. By the end of the Paleocene, most of the basin was filled, setting the stage for subsequent Tertiary sedimentation to overtop the Barrow Arch and begin significant growth of the Beaufort shelf.

 

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.

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