U.S. Geological Survey geologist Dave Houseknecht unveiled some stunning new surface maps of eastern NPR-A at the Alaska Geological Society meeting on Nov. 18. The maps use a kaleidoscope of colors to depict in great detail the topography of the area and surface characteristics, such as sand, sandy mud, vegetation and water. And these maps form the first tangible products of a project to study and document the Quaternary geology and surface features of the eastern NPR-A area, Houseknecht said.
“This project started about a year and a half ago with funding primarily from BLM and a little bit of USGS money,” Houseknecht told the geological society audience. Houseknecht sees the project as an excellent opportunity for the USGS and BLM to collaborate on some important work.
Detailed digital maps
Houseknecht expects the wealth of detail in the new maps to help land managers and others in doing environmental assessments and other planning work. And because the USGS is publishing the maps in digital format, people will be able to load the maps into geographic information computer systems.
As a first pass at preparing the maps, the USGS team digitized existing paper-based maps that USGS geologist Dave Carter completed about 20 years ago.
The USGS team has also produced maps from satellite and aircraft remote sensing data.
“The second (project) task was to evaluate whether or not we could use remote sensing to do a better job resolution-wise in the eastern NPR-A,” Houseknecht said.
The team is focusing on the Harrison Bay quadrangle in eastern NPR-A, where the current surface map has a scale of 1:250,000. That scale translates roughly into a 125-meter resolution, Houseknecht said. The team has been able to use data from the satellite based ASTER and Landsat remote sensing systems to map land surface characteristics at resolutions from 15 to 30 meters in one part of the quadrangle.
The resolution on these maps still looks a little coarse, he said. So the team has tried combining the ASTER and Landsat data with high-resolution elevation data from an IFSAR aircraft-based remote sensing survey.
“It (IFSAR) provides surface elevation data in digital format with resolutions ranging between one and five meters,” Houseknecht said.
The resulting map overlays surface characteristics on finely detailed topographic features such as narrow ridges and lake-filled depressions.
Geological insights
Geological investigations by the USGS team are providing new insights into how the current terrain of the NPR-A area formed. The team has built on previous work in the area by doing detailed surveys at locations where riverbanks or thaw lake blowouts expose sediments up to 2 million years old. The surveys delineated a widespread area of desert sands above river borne sediments and below more recent soils.
The desert sands formed during a cold period when ancient glaciers tumbled down from the Brooks Range. The team found that the sands lie as flat sheets, rather than in the long sand dunes that everyone expected.
“This was probably the biggest surprise that arose from our visit to the field this summer because when you look at the satellite and digital images what strikes you are the long linear sand dune (like) features,” Houseknecht said.
The USGS team now speculates that these linear features may be relics of rivers that once cut down into the sands.
The hunt for gravel
Houseknecht hopes that the geological investigation will shed light on where to find gravel for pads and other infrastructure in NPR-A. Of particular significance is an ancient shoreline which seems to have run well south of the current shoreline of the Beaufort Sea. The river-borne sediments to the south of that ancient shoreline present the best bet for finding gravel.
“So we are all starting to focus … on hunting for gravel south of that (shore) line,” Houseknecht said.
Gravel should also be more common in the older sediments, where evidence indicates that rivers flowing out of the ancient Brooks Range glaciers carried coarse sediments across the coastal plain.
The team has plotted the distribution of some gravel in northeast NPR-A by examining log data from old seismic shot holes — it’s possible to use the shot hole data to piece together shallow cross-sections of the subsurface sediments. In one interesting example, the team traced a deposit of sand and gravel close to the surface near the location of a proposed pad.
Work by the USGS team continues. The focus now is to clarify whether the elongated features seen on satellite images do represent ancient rivers and whether the distribution of river sediments can point to the location of gravels, Houseknecht said.
Meanwhile Houseknecht anticipates early publication of the digital maps and remote sensing data.
“They will be published by the USGS, hopefully early in 2005, in both hard copy and digital formats,” Houseknecht said.
