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Chukchi shelf stratigraphy: core study reveals pre-Cenozoic bedrock.

[ILLUSTRATION OMITTED]

With the warming of the Arctic, more focus has been put on the potential for resource development off the northern coast of Alaska in the Chukchi Sea. In February 2016, USGS (US Geological Survey) released a report titled "Upper Cretaceous and Lower Jurassic Strata in Shallow Cores on the Chukchi Shelf, Arctic Alaska."

David W. Houseknecht, William H. Craddock, and Richard 0. Lease conducted the study, which was backed in part by Statoil. They studied six cores taken during USGS exploration cruises between 1977 and 1985. The cores were extracted using either rotary or vibracore technologies.

During later expeditions, USGS captured high-channel, high-resolution, shallow seismic-reflection data, which was correlated with the cores. Since then, several newer seismic studies of the area have been taken, adding more information to the physical layers recovered in the samples.

65 Million Years and Older

The sample area is in the Chukchi Sea off the northwest corner of the North Slope. The Chukchi Shelf is a major feature of the area. Its waters are relatively shallow, about 160 feet deep. The cores were drilled to study the characteristics of the seafloor and the shallow subsurface layers of rock.

Most of the cores were taken in a region called the Hannah Trough, a Jurassic riff basin believed to have high petroleum-charging potential. Three of the cores were sampled along the northern limit of the Cenozoic fold belt, another to the northwest. One core was taken on the western rim of the Hannah Trough, all in a formation called the Herald Arch foreland. A sixth was sampled to the west of the Wrangel-Herald Arch.

The locations from which the cores were taken were selected based on the geology revealed by single-channel, high-resolution seismic reflection data. Rotary cores were taken in 1983 and vibracores in 1985. The study analyzed them to determine the presence of fossils, potential for petroleum sources, thermal maturity, and dating. The study contained detailed descriptions of each core, compared with an image of the correlating shallow seismic data to determine the age of the layers.

The cores sample pre-Cenozoic layers, 65 million years and older. The region is thought to contain large oil and gas prospects, but despite the importance of the strata in determining the geologic framework, little has been published regarding the pre-Cenozoic bedrock.

Five of the cores contain layers about 86 million to 100 million years old, in the Late Cretaceous. According to seismic data, these were interrupted by subtle angular unconformities, the Paleogene mid-Brookian unconformity in one and the Pliocene-Pleistocene in four. An unconformity is a gap in the geologic record caused by erosion, variations in sea level, or crustal deformation.

The Upper Cretaceous layers were likely deposited in freshwater coastal plain or shallow, muddy, near-shore marine environments. They are believed to be widespread, covering most of what is now Arctic Alaska.

A sixth core contains Toarcian marine organisms from the Lower Jurassic, which opens a new window into the geology of the Herald Arch region.

Four General Rock Groups

The strata were buried and put under pressure to high levels of thermal maturity during the Early Cretaceous and uplifted during the Late Cretaceous. The uplift of the Herald thrust belt added sediment to the region during the Late Cretaceous.

The study area is west of five exploration wells: Burger, Crackerjack, Diamond, Klondike, and Popcorn, which lie roughly north and east of the study area.

The seafloor of the Chukchi Shelf shows similar rock layers as those found across the Alaska North Slope, but since it is offshore, no rock exposure is present. Only five widely spaced petroleum exploration wells have been drilled. Those wells, however, were positioned on higher features and did not penetrate some layers of rock inferred to be present based on seismic reflection data and from onshore wells.

The stratigraphy shows four general groups of rocks: Brookian, Rift, Ellesmerian, and Franklinian, with several large unconformities. These subsets represent major tectonic horizons in the evolution of the region as it relates to the creation of petroleum.

The Brookian layer is composed of several distinct shale layers. The Sagavanirktok Formation lies near the top in the upper Brookian layer, underlain by an unconformity at the start of the Cenozoic era, 65 million years ago. A layer of Upper Cretaceous shale is interrupted by the Cenomanian unconformity about 100 million years ago, underlain by the Nanushuk Formation and Torok Formation. The Brookian unconformity in the early Cretaceous overlies a pebble shale unit and gamma-ray zone.

The Rift or Beaufortian is marked by the Jurassic unconformity, overlain by a layer of sandstone, the Kuparuk Formation and upper Kingak shale and lower Kingak shale.

Starting about 200 million years ago, the Sag River formation, a layer of sandstone, forms the beginning of the Ellesmerian sequence. The Shublik shale layer and Sadlerochit Group, layers of sandstone and shale, are interrupted by the Permian unconformity, which divides the Ellesmerian into upper and lower sequences.

Ellesmerian layers date from the late Devonian through the Triassic, a period from about 350 million to 200 million years ago. Limestone and dolomite form the lower strata.

The basement rocks (Franklinian), as in most of northern Alaska, are composed of meta-sedimentary rocks dating to the Devonian period. They have little to no petroleum potential.

Two layers of particular interest that were not seen in the cores are the Jurassicera lower Kingak Shale seen in outcroppings in the Brooks Range foothills and North Slope exploration drilling. Seismic data indicate Kingak Shale is present in high accommodation areas of the Chukchi Shelf such as Hanna Trough.

Tephra found in the cores is likely from volcanic ashfall on land, likely from one event in the late Cretaceous era.

Herald Arch Foreland Cores

The cores taken in the Herald Arch Foreland indicate the presence of Upper Cretaceous strata across a broad portion of the Chukchi Shelf and indicate they are widely distributed across the western part of Arctic Alaska. It is likely, the study says, Upper Cretaceous strata were present across all of Arctic Alaska before it was eroded during the Tate Cretaceious or Cenozoic eras.

The Upper Cretaceous strata is also present beneath a significant unconformity in each of the core samples. One core is truncated by the mid-Brookian unconformity, a subtle angular unconformity. Upper Cretaceous strata beneath the unconformity dip one to two degrees to the north, while the Paleogene strata above are nearly horizontal. It documents regional deformation during the Late Cretaceous or Early Paleogene about 65 million years ago.

In other cores Upper Cretaceous Strata are truncated by the Pliocene-Pleistocene unconformity. Upper Cretaceous strata are not found in the western part of the Arctic Slope, possibly due to Cretaceous to early Cenozoic uplift or later glacial erosion.

The Herald Arch core that showed the presence of Lower Jurassic marine strata offered more information of the region's geology. At the location the core was taken, Jurassic and older strata are missing in footwall, either because it wasn't deposited there or because of erosion. The strata in the core are from the hanging wall of the Herald thrust fault. The strata must have been deposited to the south and relocated by the Herald thrust fault. Dark gray shale from the Lower Jurassic strata suggest it was likely source rock material. It likely reached thermal maturity in the Late Jurassic or Early Cretaceous. It is unknown if it was oil or gas-prone.

Although the study did provide new information about the presence, spatial distribution, age, and depositional environments of the Upper Cretaceous strata on the Chukchi Shelf, they do not reveal the overall thickness of the strata or the relationship of the layers with the Nanushuk Formation.

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Diagram showing generalized chronostratigraphy of the Chukchi Shelf, Alaska. Column on right shows tectonostratigraphic sequences defined by Lerand (1973) and Hubbard and others (1987). Strata highlighted by side brackets are main focus of this report. BU, Brookian unconformity; CU, Cenomanian unconformity; JU, Jurassic unconformity; LCU, Lower Cretaceous unconformity; MBU, mid-Brookian unconformity; PPU, Pliocene-Pleistocene unconformity; PSU-GRZ, pebble shale unit and gamma-ray zone; PU, Permian unconformity; TAB, top acoustic basement.

Julie Strieker is a journalist living near Fairbanks.
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Title Annotation:SPECIAL SECTION: Petroleum Geology
Comment:Chukchi shelf stratigraphy: core study reveals pre-Cenozoic bedrock.(SPECIAL SECTION: Petroleum Geology)
Author:Stricker, Julie
Publication:Alaska Business Monthly
Geographic Code:0ARCT
Date:Jun 1, 2016
Words:1356
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