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Abstract Eight short stratigraphic sections within the Moenkopi Formation of northeastern Arizona verify a network of magnetic polarity stratigraphy previously observed in that area. The magnetostratigraphic signature was used to test the time relationships of numerous vertebrate faunal occurrences and to test the synchroneity of a major change in facies over this part of the depositional basin. The magnetic polarity data show that the vertebrate occurrences are not all of the same age; two or three different ages of fauna are indicated. Parallelism of the changes of magnetic polarity and lithology indicate a relatively rapid spread of sand-laden streams across the depositional basin. The near time-synchroneity of this lithologic change over much of the area indicates that the revised definition of the boundary between the Moqui and Holbrook members of the Moenkopi Formation as the first occurrence of a persistent, ledge-forming sandstone (Purucker and others, 1980) is a well-founded definition. A paleopole position was calculated from those samples that exhibited demagnetization behavior that is univectorial to the origin of orthogonal axes plots; this early Middle Triassic (early Anisian) paleopole is located at 94.8°E, 58.5°N (alpha-95 = 3.4°). Global correlation of the magnetostratigraphic pattern of the Moenkopi Formation to the patterns of two marine sequences indicates that Moenkopi deposition began in the Early Triassic mid-Griesbachian Stage and continued until it probably was interrupted by a hiatus that represents much of the Smithian. Deposition resumed in late Smithian and continued until late Spathian. The latest Spathian is represented by a hiatus. Deposition resumed again in the early Middle Triassic (early Anisian), thus the Moenkopi Formation provides a heretofore unknown record of geomagnetic field polarity during this time interval. A widespread Smithian hiatus, i.e., a lowstand, is suggested by comparisons of observations in south China, the Moenkopi Formation, and the Chugwater Group to those of the Arctic stratotypes.

The U.S. Geological Survey and Iowa Department of Natural Resources Geological Survey Bureau completed, in 1992, a two-year core drilling program in the Manson Impact Structure, a 35-km-diameter, Cretaceous-Tertiary boundary-age feature located in north-central Iowa. A total of 12 cores sampled in excess of 1,200 m (4,000 ft) of crater rocks, supplementing the two previously drilled shallow cores from the structure. The cores penetrated the three major terranes in the Manson Impact Structure, the terrace terrane, crater moat, and central peak. Preliminary interpretations identified several important impact-related lithologies in these cores. The most widespread is sedimentary clast breccia, a postimpact polymictic breccia or mixtite that mantles at least part of all crater terranes. Two types of crystalline clast breccias were cored on the central peak, one with a melt matrix, the other with a matrix dominated by silt- to sand-sized grains. Large blocks of basement gneiss that formed the interior of the central peak were encountered in several cores. Within the terrace terrane, structurally preserved Cretaceous strata and an overturned ejecta flap of Proterozoic and Paleozoic sedimentary rocks were encountered. Only sedimentary clast breccia was encountered in the crater moat. The preliminary investigation of these cores has provided significant information on the geometry and history of the Manson Impact Structure, but it has also prompted many more questions.

Approximately 90 Earth-crossing asteroids had been discovered through September 1989. Discovery is thought to be complete at absolute V magnitude (H) = 13.2 (the magnitude of the brightest known object, diameter ∼8.1 km), and about 6 percent complete at H = 17.7 (typical diameter about 1 km). The calculated mean probability of collision of Earth-crossing asteroids with Earth is (4.2 ± 1.7) × 10 −9 yr −1 . When multiplied by the estimated population of 1030 ± 470 at H = 17.7, this probability yields a collision rate of (4.3 ± 2.6) × 10 −6 yr −1 for asteroids larger than about 1 km in diameter. At H = 15.8, roughly equivalent to asteroid diameters more than 2 km, the estimated collision rate is ≈7 × 10 −7 yr −1 , and at 8-km diameter, the rate is ≈3 × 10 −9 yr −1 . Comet nuclei with diameters more than 2.5 km are estimated to strike the Earth at the rate of ≈ 10 −7 yr −1 ; comets larger than 10 km in diameter probably strike at a rate ≈10 −8 yr −1 . Impact of asteroids probably dominates the production of craters smaller than 30 km in diameter, whereas comet impact probably forms most craters larger than 50 km. The production rate for craters larger than 20 km in diameter, estimated from the astronomical evidence, is (4.9 ± 2.9) × 10 −15 km −2 yr −1 ; this rate is consistent with the cratering rate estimated by Grieve from the geologic record for the last 120 m.y.

At the end of 1980, seven complete cores were recovered from a 30-m (100-ft) interval in the Raton Formation at York Canyon, New Mexico. The interval cored spans the palynologically defined Cretaceous-Tertiary boundary, which is marked by a distinctive noble metal–bearing claystone in the Raton basin. Azimuthal orientation of the cores can be recovered both from the average directions of the most stable components of the remanent magnetization, with a root mean square error of 28°, and from the average direction of secondary components of magnetization removed by thermal and alternating field demagnetization, with a root mean square error of 33°. The natural remanent magnetization of about 95 percent of the core is dominated by a secondary normal polarity component. Polarity of the characteristic magnetization of each core, interpreted from 12 to 14 samples per core run, is reversed. No evidence of normal polarity characteristic magnetization was found in the 30-m (100-ft) interval sampled. The characteristic magnetization probably is a depositional remanent magnetization acquired during chron 29r. The noble metal–bearing boundary claystone in the Raton basin is interpreted to be part of a synchronous global deposit laid down at the end of the Cretaceous period.

Abstract From the south: From Henderson, Nevada, take Lake Mead Drive (Nevada 147). At 7.1 mi (11.4 km) east of U.S. 95, turn north onto Northshore Road and go 3.2 mi (5.1 km) to Lake Mead Boulevard. The site is 3.7 mi (6 km) north on Lake Mead Boulevard. From the north: From North Las Vegas, take Lake Mead Boulevard (also Nevada 147) east from 1–15. At 11.4 mi (18.3 km) from 1–15 a paved road from the east (left) intersects Nevada 147. Continue on 147 for 1.8 mi (2.9 km) to the site. The site is marked by the high-tension power line that crosses the road. Park along the side of the road or on the dirt roads that intersect the main road in this area.

Abstract The location of Meteor Crater is shown on nearly all high-way maps of Arizona. It lies 6 mi (9.7 km) south of 1-40 between Flagstaff and Winslow in northern Arizona; the turnoff from 1-40, 34 mi (55 km) east of Flagstaff, is well marked by signs along the highway.Access to the crater is by a paved road that leads directly to a visitor center and museum on the rim of the crater. Qualified scientists may obtain permissio to hike to various parts of the crater by writing or calling in advance to Meteor Crater Enterprises, 121 East Birch Avenue, Flagstaff, Arizona 86001.