Preliminary interpretation of industry two-dimensional seismic data from Susitna Basin, south-central Alaska

Abstract

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit http://www.usgs.gov or call 1-888-ASK-USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit http://store.usgs.gov Suggested citation: Lewis, K.A., Potter, C.J., Shah, A.K., Stanley, R.G., Haeussler, P.J., and Saltus, R.W., 2015, Preliminary interpretation of industry two-dimensional seismic data from Susitna Basin, south-central Alaska: U.S. Geological Survey Open-File Report 2015-1138, 51 p., http://dx.doi.org/10.3133/ofr20151138. ISSN 2331ISSN -1258 Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. AbstractLocated approximately 80 kilometers northwest of Anchorage, Alaska, the Susitna Basin is a complex sedimentary basin whose tectonic history has been poorly understood. Recent interpretation of two-dimensional seismic reflection data integrated with well, aeromagnetic, and gravity data provides new insights into the structural and stratigraphic nature of the basin.This report presents an interpretation of 41 two-dimensional seismic reflection lines, acquired by industry from the 1960s to the 1980s. Our interpretation of the seismic data focused mainly on picking two Eocene stratigraphic units and a presumed base of Tertiary horizon. Based on our interpretation of the seismic data, the structural features in the basin appear to be generally contractional, as evidenced by the presence of many reverse faults, thrust faults, and folds, with the contraction mainly oriented east-west. This result is contrary to prior inferences of most previous geologic studies that showed normal faults. Several regional reverse faults have been identified in the seismic data and appear to divide the basin into three regions or "sides": east, west, and south.The eastern seismic lines show evidence of numerous short-wavelength antiforms that appear to correspond to a series of northeast-trending lineations observed in aeromagnetic data, which have been interpreted as being due to folding of Paleogene volcanic strata. The eastern side of the basin is also cut by a number of reverse faults and thrust faults, the majority of which strike north-south. The western side of the Susitna Basin is cut by a series of regional reverse faults and is characterized by synformal structures in two fault blocks between the Kahiltna River and Skwentna faults. These synforms are progressively deeper to the west in the footwalls of the east-vergent Skwentna and northeast-vergent Beluga Mountain reverse faults. Although the seismic data are limited to the south, we in...

Late Oligocene to present contractional structure in and around the Susitna basin, Alaska—Geophysical evidence and geological implications

“…23 Ma inferred from thermochronologic data by Benowitz et al (2012a, p. 13). The thickest Neogene deposits in the Susitna depocenter, ~2500 m thick in the Trail Ridge Unit 1 well, occur in a broad synform that is bounded on its western and eastern margins by north-striking reverse faults (Stanley et al, 2014;Lewis et al, 2015). We hypothesize that Neogene subsidence and deposition in the synform, as well as movement along the reverse faults, resulted from contractional deformation associated with Yakutat microplate subduction.…”

“…1-3). In the Susitna depocenter, recent investigations using data from exploratory wells, seismic-reflection, gravity, and aeromagnetic data (Shah et al, 2014;Stanley et al, 2014;Lewis et al, 2015) indicate that a deeply buried Paleogene sequence, known primarily from two deep exploratory wells (the Pure Kahiltna Unit 1 and Trail Ridge Unit 1; see Fig. 2), is unconformably overlain by a Neogene and Quaternary sequence that is present in several exploratory wells and scattered surface outcrops ( Gillis et al, 2013).…”

“…We hypothesize that Neogene subsidence and deposition in the synform, as well as movement along the reverse faults, resulted from contractional deformation associated with Yakutat microplate subduction. It is possible that the north-striking Neogene reverse faults seen on seismic-reflection profiles (Lewis et al, 2015) are reactivated Paleogene normal faults, but this hypothesis remains to be evaluated. Seismic-reflection and aeromagnetic data show that Paleogene strata in the eastern part of the Susitna depocenter are folded and cut by reverse and thrust faults (Shah et al, 2014;Lewis et al, 2015); the timing of the folding is unclear, but at least some of the northeast-striking faults appear to have surface expression and therefore may be geologically young.…”

“…It is possible that the north-striking Neogene reverse faults seen on seismic-reflection profiles (Lewis et al, 2015) are reactivated Paleogene normal faults, but this hypothesis remains to be evaluated. Seismic-reflection and aeromagnetic data show that Paleogene strata in the eastern part of the Susitna depocenter are folded and cut by reverse and thrust faults (Shah et al, 2014;Lewis et al, 2015); the timing of the folding is unclear, but at least some of the northeast-striking faults appear to have surface expression and therefore may be geologically young. Geophysical evi dence, discussed in detail in this report, indicates that the southwestern margin of the Susitna depo center is the Beluga Mountain fault, a northweststriking, southwest-dipping thrust fault.…”

“…Existing structural information for the basin is sparse; aside from the Castle Mountain fault, none of the bounding structures are exposed. Subsurface data consist of a modest number of vintage seismic lines (e.g., Lewis et al, 2015) and a few wells, none of which drilled deep enough to reach crystalline basement. Other data include 1970s and newer gravity data, mostly collected by the State of Alaska and the U.S. Geological Survey (USGS; see Supplemental File 1 ) and aeromagnetic data from surveys collected in 2000 and 2012 by the USGS (http:// mrdata .usgs .gov /magnetic /show -survey .php?id = 4247, http:// mrdata .usgs .gov /magnetic /show -survey .php?id = 10001).…”