Seismic analysis of clinoform depositional sequences and shelf‐margin trajectories in Lower Cretaceous (Albian) strata, Alaska North Slope

Abstract

Lower Cretaceous strata beneath the Alaska North Slope include clinoform depositional sequences that filled the western Colville foreland basin and overstepped the Beaufort rift shoulder. Analysis of Albian clinoform sequences with two‐dimensional (2D) seismic data resulted in the recognition of seismic facies inferred to represent lowstand, transgressive and highstand systems tracts. These are stacked to produce shelf‐margin trajectories that appear in low‐resolution seismic data to alternate between aggradational and progradational. Higher‐resolution seismic data reveal shelf‐margin trajectories that are more complex, particularly in net‐aggradational areas, where three patterns commonly are observed: (1) a negative (downward) step across the sequence boundary followed by mostly aggradation in the lowstand systems tract (LST), (2) a positive (upward) step across the sequence boundary followed by mostly progradation in the LST and (3) an upward backstep across a mass‐failure décollement. These different shelf‐margin trajectories are interpreted as (1) fall of relative sea level below the shelf edge, (2) fall of relative sea level to above the shelf edge and (3) mass‐failure removal of shelf‐margin sediment. Lowstand shelf margins mapped using these criteria are oriented north–south in the foreland basin, indicating longitudinal filling from west to east. The shelf margins turn westward in the north, where the clinoform depositional system overstepped the rift shoulder, and turn eastward in the south, suggesting progradation of depositional systems from the ancestral Brooks Range into the foredeep. Lowstand shelf‐margin orientations are consistently perpendicular to clinoform‐foreset‐dip directions. Although the Albian clinoform sequences of the Alaska North Slope are generally similar in stratal geometry to clinoform sequences elsewhere, they are significantly thicker. Clinoform‐sequence thickness ranges from 600–1000 m in the north to 1700–2000 m in the south, reflecting increased accommodation from the rift shoulder into the foredeep. The unusually thick clinoform sequences suggest significant subsidence followed by rapid sediment influx.

Influence of the Kingak Shale ultimate shelf margin on frontal structures of the Brooks Range in the National Petroleum Reserve in Alaska

“…The initial well ties were based on the interpretations of previous workers (for example, Molenaar, 1988;Bird and Molenaar, 1992;Houseknecht and Bird, 2004;Kulander and others, 2005;Houseknecht, Bird, and Schenk, 2009) and correlation between distinct seismic reflections and well-log response. These well ties subsequently were refined by construction of a synthetic seismogram for each well for which both sonic and density logs were available, as well as by correlation of the resultant synthetic to nearby seismic lines.…”

“…The Beaufortian sequence is a succession of Jurassic to Lower Cretaceous (Neocomian) siliciclastic strata that record erosional and depositional effects of rift opening of the Canada Basin (Hubbard and others, 1987;Bird, 2004, 2011). The Brookian sequence consists of Cretaceous to Cenozoic siliciclastic sediments derived from the Chukotka and Brooks Range orogenic belts and deposited as deep marine through nonmarine strata in the Colville foreland basin and on the continental margin to the north (Houseknecht, Bird, and Schenk, 2009;Houseknecht and Bird, 2011). Regional aspects of these four sequences have been summarized by Moore and others (1994), Bird (2001), and Bird and Houseknecht (2011).…”

“…2) and consists of interbedded mudstone, sandstone, and coal deposited in a spectrum of shallow marine to nonmarine environments. Although internally it appears seismically tabular, the topset element thins eastward and northward across NPRA (Bird, 1988) as the result of accommodation growth in the foredeep during deposition and the aggradational trajectory of the toplap surface that separates shallow marine facies above from marine-slope facies below (Houseknecht and Schenk, 2001;Houseknecht, Bird, and Schenk, 2009;Houseknecht, LePain, and others, 2009). …”

“…The lowest element (lower part of Torok Formation) is a wedge-shaped body of mostly sandstone deposited in a basin-floor fan setting and restricted to the foredeep ( fig. 2; Houseknecht and Schenk, 2001;Houseknecht, Bird, and Schenk, 2009;Houseknecht, LePain, and others, 2009). This "foredeep wedge" is at least 2 kilometers (km) thick in the area of greatest accommodation, which is basinward of the Kingak ultimate shelf margin (fig.…”

“…The Torok and Nanushuk Formations, together with the GRZ, constitute a clinothem formed by a depositional system that prograded eastward and northward in the Colville foreland basin (Molenaar, 1988;Houseknecht, Bird, and Schenk, 2009). The Torok Formation, mostly Aptian-Albian in NPRA, comprises sandstone and silty mudstone deposited in basin-floor fan and marine slope environments.…”