Geology, geophysics, and geochronology of the Manson impact structure

Abstract

A problem with the impact hypothesis for the Cretaceous/Tertiary (K/T) mass extinction is the apparent absence of an identifiable impact site. The Manson impact structure is a candidate site because of its size (the largest such structure recognized in the United States); in addition, the largest and most abundant shocked quartz grains at the K/T boundary are found relatively close by, and its age is indistinguishable from that of the K/T boundary.The region of northwest central Iowa that contains the Manson impact structure is covered by Quaternary glacial deposits, which are underlain by Phanerozoic sedimentary rocks (mostly flat-lying carbonates) and Proterozoic red clastic, metamorphic, volcanic, and plutonic rocks. In a circular area about 22 mi (35 km) in diameter around Manson, Iowa, this normal sequence is absent or "disturbed." Within the structure, three roughly concentric zones of rock associations have been identified: an outer zone of displaced strata, a zone of completely disrupted strata, and a central area in which basement igneous and metamorphic rocks have been uplifted at least 1,220 ft (4,000 m). Gravity, magnetic, and seismic refraction surveys readily identify the central uplift within the structure. Manson is established as an impact structure based on its circular shape, its central uplift, and the presence of shocked quartz within the granitic central uplift.Paleontological evidence, a fission track age, and preliminary 40 Ar/ 39 Ar dating all allowed a K/T boundary age for the Manson structure. Improved 40 Ar/ 39 Ar age spectra may be interpreted in terms of samples that were incompletely degassed during heating due to the Manson impact. An age of 65.7 ±1.0 Ma was obtained, a value indistinguishable from that of the K/T boundary.

Petrography of crystalline clast breccias from the Manson M-1 core

“…Interest in the MIS increased when shock-metamorphosed feldspars from the central peak yielded a series of 40 Ar/ 39 Ar ages indistinguishable from that of the CretaceousTertiary boundary (65.3 ± 0.5 to 65.7 ± 1 Ma; Hartung et al, 1986;Kunk et al, 1989;Zeitler and Kunk, 1993), although more recent 40 Ar/ 39 Ar dating of sanidine by laser fusion indicates that the MIS is significantly older (73.8 ± 0.3 Ma; Izett et al, 1993). Preliminary descriptions of the geology and petrology of the MIS were presented by Hartung et al (1990), Koeberl and Hartung (1992), and Anderson and Hartung (1992).…”

“…This unusual occurrence of crystalline rocks was attributed to volcanic activity or a bolide impact (Hartung et al, 1990). Interest in the MIS increased when shock-metamorphosed feldspars from the central peak yielded a series of 40 Ar/ 39 Ar ages indistinguishable from that of the CretaceousTertiary boundary (65.3 ± 0.5 to 65.7 ± 1 Ma; Hartung et al, 1986;Kunk et al, 1989;Zeitler and Kunk, 1993), although more recent 40 Ar/ 39 Ar dating of sanidine by laser fusion indicates that the MIS is significantly older (73.8 ± 0.3 Ma; Izett et al, 1993).…”

“…These rocks are intruded and overlain by ca. 1,280-and 1,000-Ma diabase dikes and basaltic flows (Van Schmus et al, 1990;Hartung et al, 1990).…”

“…The crystalline basement complex includes Penokean (1,890-1,850 Ma) porphyroblastic magnetite gneiss and oligoclase-biotite-quartz gneiss (Hartung et al, 1990), as well as amphibolite and biotiteamphibolite gneisses encountered in the M-1, M-5, M-7, and M-8 cores from the flank of the central peak, and the M-10 core from the middle of the central peak (see Fig. 1).…”

The Edgerton Structure: A Possible Meteorite Impact Feature in Eastern Kansas

“…In fact, Big Basin and Little Basin in Clark County in the southwestern part of the state have been cited as examples (e.g., [2]). Several impact features have been identified and verified in the U.S. Midcontinent, for example, Manson in Iowa (Cretaceous; [3,4]), Ames in Oklahoma (Cambrian; [5]), Haswell Hole in Colorado (Precambrian; [6,7]), and possibly Belton RingFault Complex in Missouri (post-Pennsylvanian; [8]) and Merna in Nebraska (Recent; [9]). So why not in Kansas?…”