THE TERMINAL CRETACEOUS MANSON IMPACT STRUCTURE OF IOWA AND ITS POSSIBLE INFLUENCE ON K-T BOUNDARY EVENTS IN THE WESTERN INTERIOR

by
B.J. Witzke and R.R. Anderson

The Marshal Lambert Symposium Program of Events
North Dakota Geological Survey and Pioneer Trails Museum, p. 13-16

ABSTRACT


The Manson Impact Structure of northern Iowa is the largest well preserved impact structure in the United States (35 km diameter). Its 40Ar/39Ar age of 65.4 0.4 Ma (Kunk et al., 1993) is indistinguishable from the age of the K-T boundary. Although it now seems likely that multiple impact events occurred at or near the end of the Cretaceous, including the probable giant structure at Chicxulub, Yucatan (Hildenbrand et al., 1991), the Manson structure is of special interest because of its geographic proximity to K-T boundary sections in the North American Western Interior. Manson is considered a likely source for some of the impact ejecta identified in the iridium-enriched impact layer of the K-T boundary layer in the Western Interior (Shoemaker and Izett, 1992). Maximum sizes of shocked quartz grains in K-T boundary layers worldwide generally decrease with increasing distance from Manson. A recent drilling program in the Manson structure, co-sponsored by the U.S. Geological Survey and the Iowa Department of Natural Resources - Geological Survey Bureau, has identified a variety of impact-related rock types (Anderson et al., 1994) and refined our understanding of the general internal structure of the feature (see also Hartung and Anderson, 1988, for a synthesis of previous studies). Three major structural terranes are identifed: 1) a broad Central Peak of Precambrian basement rocks uplifted 6-7 km; 2) an outer Terrace Terrane of complex downdropped blocks, including Cretaceous stratigraphic units structurally preserved far to the east of their present-day erosional margins across the Dakotas; and 3) an intermediate Crater Moat. An overturned ejecta flap of stratigraphically inverted Proterozoic and Phanerozoic blocks/clasts is identified in one drill core from the terrace terrane, in a region downdropped at least 600 m relative to the crater margin. Crater modeling indicates that faulting within the terrace terrane and uplift of the central peak would have occurred within seconds to minutes after impact.

Impact-related rocks include two general types of breccias: 1) crystalline clast breccias (CCB) dominated by clasts of Precambrian basement lithologies (gneiss, granite, minor mafics) and incorporating clasts of accretionary melt rock material (some with isotropic domains of apparently largely unaltered glass) and minor Proterozoic sedimentary rocks; and 2) sedimentary clast breccias (SCB) dominated by clasts of Phanerozoic shales and carbonates. CCBs were identified blanketing the crystalline basement rocks of the Central Peak, and display contrasting matrix lithologies: 1) a lower interval with a sandy argillaceous matrix, and 2) an overlying unit with an altered melt-rock matrix displaying regions of flow-banding (interpreted as the impact melt layer). Clasts and grains within the CCBs show abundant planar deformation features (PDFs), especially quartz with multiple sets of shock lamellae, features unique to impact events. Some clasts are partially to completely melted, and many are modified by early post-impact hydrothermal alteration (Crossey and McCarville, 1993). The CCBs overlie relatively intact large blocks of Precambrian basement rocks on the central peak, which incorporate pods or dikes of CCB and display thin pseudotachylite (frictional melt) veins generated along fracture offsets. Mineral crystals in these basement rocks also display abundant PDFs. By contrast, the SCBs are known to blanket all three structural terranes in the crater. The most abundant SCB contains only scattered rare clasts of Precambrian basement lithologies, and melt-rock clasts are rare to absent. These breccias, however, contain a disproportionately high percentage of Cretaceous shale, mudstone, and sandstone clasts, including contorted large blocks up to 75 m in diameter. Isolated well-preserved Cretaceous foraminifera tests are scattered to common in the SCB matrix. A layer of SCB was deposited across the Central Peak and is preserved in a central depression where it locally incorporated clasts of impact melt-rock into its base.

The disproportionate dominance of Cretaceous lithologies in the SCBs, when compared to the total volume of crustal material excavated during transient cratering, suggests that much of the contained Cretaceous blocks and other material were derived by sedimentary mass transport from the crater margin, not by ejecta fall-back. Unstable scarps probably existed around the crater margin immediately following excavation of the transient crater and as unstable footwall scarps associated with the episode of normal block faulting during formation of the Terrace Terrane area. The upper strata of these scarps was dominated by Cretaceous sediments (estimated 300 m of Cenomanian through Maastrichtian strata) and, along with the ejecta blanket, presumably served as the source for the bulk of the SCB material. These Cretaceous blocks, clasts, and matrix provide a record of Cretaceous deposits now largely eroded from the eastern margin area of the Western Interior. The presence of Paleozoic and rare Proterozoic and melt-rock lithologies identify the ejecta component of the SCB, incorporated chaotically with the mass of transported Cretaceous material during emplacement. Therefore, it is suggested that the SCB was transported from the crater rim by gravitationally driven mass wasting, coincident with and immediately following large-scale fault movements associated with the formation of the Terrace Terrane and the uplift of the central peak. This emplacement of the SCB included the transport of material, in thickness known to exceed 200 m, up to 15 km or more laterally, to cover all regions including the center of the Manson structure.

The scale of movement associated with emplacement of the SCB is hard to comprehend, and it is unclear if a gravity-driven landslide alone was sufficient to accomplish the task. This raises some intriguing questions: was the Manson site dry land at the time of impact, or was the site covered by a shallow seaway at that time? Some workers (e.g., Feldman, 1972) have raised the possibility that the Western Interior seaway did not completely withdraw at the end of the Cretaceous, noting that the eastward-thinning Hell Creek-Ludlow nonmarine package is sandwiched between marine units above and below (Pierre-Fox Hills, Cannonball), similar to other Cretaceous nonmarine wedges (e.g., Judith River). If the Hell Creek thinning trend is extrapolated eastward beyond the present-day erosional edge, it may be possible that the Western Interior seaway remained in the eastern Dakotas and western Iowa at K-T boundary time. If the Manson impact occurred in a shallow sea, tsunami-like currents may have swept back into the crater following impact, possibly transporting enormous amounts of material.

What effects might be predicted to have occurred at the end of the Cretaceous in the Western Interior as a result of the nearby Manson impact? First, based on tentative scaling of known blast zones against the calculated energy released from a Manson-size impact, about 2.2 x 1021 Joules (Anderson and Hartung, 1992), the effects of the atmospheric shock wave generated from the Manson blast would have been dramatic. All combustible material within about 200 km of Manson would have been ignited, standing vegetation completely devastated out to a distance of approximately 600 km, and the shock wave could have knocked human-sized animals off their feet as far away as 1,300 km. The force of the blast was sufficient to kill most exposed terrestrial animals in its wake as far as 1,000 km. These distances intersect various K-T boundary sections in the Western Interior, including areas in south-central North Dakota (650-750 km), eastern Wyoming [including Teapot Dome] (800-950 km), southwest North Dakota [including the Bowman area] (850 km), the Raton Basin (1,050-1,100 km), and the type Hell Creek area (1,100 km). Second, significant amounts of ejecta were generated from the Manson impact, with estimates ranging from 600 to 1,200 cubic kilometers (Anderson and Hartung, 1992; Roddy and Shoemaker, 1993), about 15% of which would have been injected above the tropopause. The impacting body (comet or asteroid) completely vaporized on impact, providing a source for distribution of extraterrestrial material, likely iridium-enriched. Therefore, it seems reasonable to suggest that the Manson impact contributed ejecta (including large amounts of shocked quartz) and extraterrestrial material to K-T boundary sections in the Western Interior and elsewhere in the world (Anderson and Hartung, 1992). However, if multiple impacts occurred around K-T boundary time, Manson would only provide a portion of this material.

Finally, what effects might the Manson impact, and other K-T impacts, have had on terminal Cretaceous extinctions? This question is certainly more difficult to ponder, as extinction processes are inadequately constrained. However, the original Alvarez et al. (1980) hypothesis of global climatic changes created by the atmospheric injection of large volumes of impact-generated material remains intriguing, as the global consequences of such a crisis on the biosphere may have been more than incidental. If a series of impacts occurred at or near the end of the Cretaceous, including that at Manson, the atmospheric effects may have been even more significant, possibly drawing out over longer periods of time or creating a succession of crises aggravated by other global changes (e.g., long-term global cooling, Deccan volcanism, eustatic sea-level fall). Although the Manson structure is significantly smaller than the current estimates of the size of the Chicxulub structure, the Manson impact remains one of the most remarkable events in the Cretaceous history of the North American Western Interior. Many new details concerning the Manson structure will be emerging in the near future as a consortium of researchers continues to study the newly drilled core materials.

 


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