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GSA Bulletin: 40Ar/39Ar Age of the Manson Impact Structure, Iowa, And 40Ar/39Ar age of the Manson impact structure, Iowa, and correlative impact ejecta in the Crow Creek Member of the Pierre Shale (Upper Cretaceous), South Dakota and Nebraska G. A. Izett* Department of Geology, College of William and Mary, 3012 East Whittaker Close, Williamsburg, Virginia 23185 W. A. Cobban U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225 G. B. Dalrymple College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331 J. D. Obradovich U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225 ABSTRACT INTRODUCTION A set of 34 laser total-fusion 40Ar/39Ar analyses of sanidine from a A buried impact structure (~35 km diameter) near Manson, Iowa, has at- melt layer in crater-fill deposits of the Manson impact structure in Iowa tracted the attention of geologists for more than 80 years. Renewed interest has a weighted-mean age of 74.1 ± 0.1 Ma. This age is about 9.0 m.y. in this structure followed the proposal of Alvarez et al. (1980) that the older than 40Ar/39Ar ages of shocked microcline from the Manson im- Cretaceous-Tertiary (K-T) boundary mass-extinction event was caused by pact structure reported previously by others. The 74.1 Ma age of the the impact of a large asteroid calculated to be about 10 km in diameter. sanidine, which is a melt product of Precambrian microcline clasts, in- Chemical and physical evidence that supports their hypothesis has been dicates that the Manson impact structure played no part in the Creta- found globally at the exact K-T boundary. The evidence includes (1) an irid- ceous-Tertiary (K-T) mass extinction at 64.5 Ma. Moreover, incremen- ium anomaly (Alvarez et al., 1980); (2) shocked quartz, feldspar, quartzite, tal-heating 40Ar/39Ar ages of the sanidine show that it is essentially free and zircon (Bohor et al., 1984, 1992; Izett and Pillmore, 1985; Izett, 1987, of excess 40Ar and has not been influenced by postcrystallization heat- 1990); (3) relic tektites (Izett et al., 1990; Izett, 1991a; Sigurdsson et al., ing or alteration. An age spectrum of the matrix of the melt layer shows 1991); and (4) Ni-rich spinel (Bohor et al., 1986; Robin et al., 1992). The effects of 39Ar recoil, including older ages in the low-temperature incre- Alvarez hypothesis triggered a search for the location of the impact, and the ments and younger ages in the high-temperature increments. At 17 most promising sites turned out to be Chicxulub on the Yucatan Peninsula places in eastern South Dakota and Nebraska, shocked quartz and (Penfield and Camargo, 1981; Hildebrand et al., 1991) and near Manson, feldspar grains are concentrated in the lower part of the Crow Creek Iowa (Izett, 1990; Anderson et al., 1996). Chicxulub has emerged as the Member of the Pierre Shale (Upper Cretaceous). The grains are largest most likely impact site, and Manson has been excluded mainly because it (3.2 mm) in southeastern South Dakota and decrease in size (0.45 mm) has been 40Ar/39Ar dated at 73.8 Ma (Izett et al., 1993b). This age for the to the northwest, consistent with the idea that the Manson impact Manson impact structure is 9.3 m.y. older than the 64.5 Ma age of tektites structure was their source. The ubiquitous presence of shocked grains (Izett et al., 1991) in a bed that precisely marks the K-T boundary on Haiti. concentrated in a thin calcarenite at the base of the Crow Creek Mem- Since Izett et al. (1993b) published their preliminary age of 73.8 Ma for ber suggests it is an event bed recording an instant of geologic time. the Manson impact structure, we have acquired a new set of 40Ar/39Ar Ammonites below and above the Crow Creek Member limit its age to analyses of sanidine from core of recent drilling in this structure. We herein the zone of Didymoceras nebrascense of earliest late Campanian age. report the ages and associated analytical data that establish a more reliable Plagioclase from a bentonite bed in this zone in Colorado has a age for the Manson impact structure. In addition, we describe shocked min- 40Ar/39Ar age of 74.1 ± 0.1 Ma commensurate with our sanidine age of eral grains in the Crow Creek Member of the Pierre Shale (Upper Creta- 74.1 Ma for the Manson impact structure. 40Ar/39Ar ages of bentonite ceous) in South Dakota and Nebraska that arguably are distal ejecta from beds below and above the Crow Creek are consistent with our 74.1 ± 0.1 the Manson impact event. Ammonite collections from below and above the Ma age for the Manson impact structure and limit its age to the inter- Crow Creek Member indicate that it was deposited in the range zone of val ±74.5 0.1 to 73.8 ± 0.1 Ma. Recently, two origins for the Crow Creek Didymoceras nebrascense of earliest late Campanian age. We also report have been proposed—eastward transgression of the Late Cretaceous 40Ar/39Ar ages for bentonite beds in the Pierre Shale just below and above sea and a Manson impact-triggered tsunami. We conclude that most the Crow Creek Member that are compatible with our age for the Manson data are in accord with an impact origin for the Crow Creek Member impact structure and provide an independent check on its numerical isotopic and are at odds with the marine transgression hypothesis. age. Two origins for the member are evaluated. *Corresponding author; e-mail: [email protected] Data Repository item 9813 contains additional material related to this article. GSA Bulletin; March 1998; v. 110; no. 3; p. 361–376; 6 figures; 2 tables. 361 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/3/361/3382908/i0016-7606-110-3-361.pdf by guest on 30 September 2021 IZETT ET AL. MANSON IMPACT STRUCTURE (1993) determined that certain Manson crater-fill rocks have normal rema- nent magnetization. Taken at face value, the data exclude the possibility that About 40 years ago, drilling and geophysical work showed that a complex the Manson impact event occurred exactly at the K-T boundary because the circular structure lies buried beneath a (~30 m) blanket of Pleistocene glacial rocks that contain the boundary are reversely magnetized. In addition, U-Pb drift in north-central Iowa. Data then available indicated that the structure ages of shocked zircons from the upper of two K-T boundary claystone lay- consisted of a relatively large central zone composed of Proterozoic granite ers in the Raton basin of Colorado are not compatible with the age of Pre- and gneiss surrounded by an outer annular zone of faulted Phanerozoic cambrian crystalline basement rocks in the Manson, Iowa, area, or sedimen- strata. The Proterozoic crystalline rocks had been uplifted at least 5 km above tary rocks derived from the erosion of such rocks (Bohor et al., 1992; Krogh their normal basement position. Hoppin and Dryden (1958) thought that the et al., 1992; Premo and Izett, 1993). Although their data set was small, Blum structure formed by cryptovolcanic processes, but Short (1966) showed that et al. (1993) determined that the isotopic composition (Rb/Sr, Nd/Sm, δ18O) crystalline rocks of its core were shock metamorphosed, the result of the im- of K-T boundary tektites from Haiti and Manson impact structure melt rocks pact of a large asteroid. To explore the structure, a 146-m-deep core hole are significantly different. (Manson 2A) was drilled in 1953 in the Proterozoic crystalline rocks near the Because the ages of Kunk et al. (1989) were suspect, it became important center of the structure (Hoppin and Dryden, 1958). This buried structure be- to obtain samples of a theoretically possible melt layer to establish a reliable came an early candidate for the K-T boundary impact site when French age for the Manson impact structure. In 1991–1992, 12 holes were drilled (1984) suggested that the impact occurred either at the Sierra Madre, Texas, in the Manson impact structure crater fill rocks to search for a possible melt or Manson, Iowa, structures. He considered Manson to be the more likely be- layer, to find fossiliferous postimpact crater-fill sediments, and to explore cause of the size (0.1 mm) of shocked minerals reported by Bohor et al. mineralogical-geochemical variations in the crater-fill rocks. One of the 12 (1984). Izett and Pillmore (1985) and Izett (1987, 1990) also suggested that drill holes (M-1, 216 m total depth) was drilled 6 km northeast of the center the Manson impact structure might be the impact site after recovering sub- of the Manson impact structure and on the flank of its Proterozoic crys- stantially larger mineral grains (0.64 mm) from K-T boundary sedimentary talline rock central peak (see Anderson et al., 1996, Fig. 3, for location of rocks in Colorado, New Mexico, and Montana. drill holes). Core from the M-1 hole proved to be the most interesting and Shoemaker and Izett (1992) speculated that the K-T boundary mass ex- promising target for isotopic age studies. tinction was caused by multiple impacts rather than a single event. Two litho- logically different claystone layers separated by a sharp contact constitute the Description of M-1 Core K-T boundary sequence at most Western Interior sites (Izett, 1990). On the basis of this evidence, Izett (1991b) proposed that altered microtektites in the Several lithologic names have been applied to the polymict breccias of lower kaolinte claystone layer originated from the Chicxulub impact struc- the M-1 core.
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