Illinois

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Fig. 1. Map showing fission-track sample locations, location of the Newton County stone quarry in Kentland, Indiana, and sampled counties in Illinois (LS-La Salle) and Indiana (C-Clark, Cl-Clay, F-Franklin, J-Jackson, L-Lake, N-Newton, and W-White). All samples collected are shown; only those listed in Tables 1-5 yielded enough apatite grains (> 5) to be analyzed and used in this study.

Fig. 1. Map showing fission-track sample locations, location of the Newton County stone quarry in Kentland, Indiana, and sampled counties in Illinois (LS-La Salle) and Indiana (C-Clark, Cl-Clay, F-Franklin, J-Jackson, L-Lake, N-Newton, and W-White). All samples collected are shown; only those listed in Tables 1-5 yielded enough apatite grains (> 5) to be analyzed and used in this study.

Fig. 2. Schematic stratigraphic column showing rock types and units exposed and drilled in and around the Newton County stone quarry, Kentland, Indiana, major unconformity-bounded transgressive-regressive Paleozoic depositional sequences, after Sloss (1963), and Pleistocene drift. Sequence names given on left are discussed in text; ages and some specific rock unit names discussed in text (S-Shakopee Formation; QM-Quimby's Mill Formation; St. Peter sandstone) are given on right. Rock types are represented by: brick pattern-limestone, diagonal brick pattern-dolostone, horizontal dashes-shale; small dots-sandstone; dots and blobs-glacial drift; the gray vertically ruled lines represent unconformities, gaps in the rock record.

Fig. 2. Schematic stratigraphic column showing rock types and units exposed and drilled in and around the Newton County stone quarry, Kentland, Indiana, major unconformity-bounded transgressive-regressive Paleozoic depositional sequences, after Sloss (1963), and Pleistocene drift. Sequence names given on left are discussed in text; ages and some specific rock unit names discussed in text (S-Shakopee Formation; QM-Quimby's Mill Formation; St. Peter sandstone) are given on right. Rock types are represented by: brick pattern-limestone, diagonal brick pattern-dolostone, horizontal dashes-shale; small dots-sandstone; dots and blobs-glacial drift; the gray vertically ruled lines represent unconformities, gaps in the rock record.

Stratigraphic relations provide broad constraints on the timing of deformation at Kentland (Fig. 2, 3). Silurian carbonates, now oriented subvertically in the quarry, and tilted Mississippian strata mapped nearby in the subsurface (Gutschick 1987), are unconformably overlain by flat-lying, undeformed Pleistocene glacial deposits. Thus, the Kentland event occurred sometime between ~300 and 1 m.y. ago.

A previous paleomagnetic study determined a post-Late Cretaceous (< 97±10 Ma) age for the Kentland deformation event (Jackson and Van der Voo 1986). This determination was based on a paleomagnetic pole position obtained from 25 remagnetized Middle Ordovician Quimby's Mill Limestone samples collected in the quarry at Kentland (Fig. 2). The remagnetized samples failed a fold test (more accurately a tilt test, because all but two of the samples dipped to the northwest), and were thus interpreted to have acquired their magnetic signatures before Kentland doming; the ~97±10 paleomagnetic Ma age was, therefore, considered by Jackson and Van der Voo (1986) to be a maximum age for the deformation.

Fig. 3. A. Bedrock geologic map for the Kentland, Indiana area, after Gutschick (1987). Map is based on Newton County stone quarry (marked quarry on map) exposures and on > 100 drill cores that reached bedrock. No bedrock exposures exist outside of quarry for the area shown, which is covered by glacial drift. The Kentland crater is thus mostly buried, deeply eroded and highly exhumed, and has only a geologic expression and no geomorphic expression. Rock units mapped are shown by age: O-Ordovician, S-Silurian, D-Devonian, M-Mississippian, P-Pennsylvanian paleostream channel deposits; Gf-Gutschick Fault; f-unnamed fault. A-A' shows line of cross-section. B. Geologic cross-section, after Gutschick (1987), showing undeformed Pleistocene glacial drift cover (thickness exaggerated) over domed Ordovician-Mississippian strata. Means fault (Mf) and Kentland quarry fault (KQf) are well exposed and mapped in quarry but not shown in Fig. 2A. Thickness of Pennsylvanian channel shown schematically.

Fig. 3. A. Bedrock geologic map for the Kentland, Indiana area, after Gutschick (1987). Map is based on Newton County stone quarry (marked quarry on map) exposures and on > 100 drill cores that reached bedrock. No bedrock exposures exist outside of quarry for the area shown, which is covered by glacial drift. The Kentland crater is thus mostly buried, deeply eroded and highly exhumed, and has only a geologic expression and no geomorphic expression. Rock units mapped are shown by age: O-Ordovician, S-Silurian, D-Devonian, M-Mississippian, P-Pennsylvanian paleostream channel deposits; Gf-Gutschick Fault; f-unnamed fault. A-A' shows line of cross-section. B. Geologic cross-section, after Gutschick (1987), showing undeformed Pleistocene glacial drift cover (thickness exaggerated) over domed Ordovician-Mississippian strata. Means fault (Mf) and Kentland quarry fault (KQf) are well exposed and mapped in quarry but not shown in Fig. 2A. Thickness of Pennsylvanian channel shown schematically.

This study outlines our attempt to use apatite fission tracks to better determine the age of the Kentland crater. Samples were collected and analyzed in three stages: 1) from the Newton County stone quarry in Kentland near the center of the crater, 2) tens of km from the crater, using subsurface cores from three surrounding counties, and 3) from distant subsurface cores and outcrops, hundreds of km from the crater in Indiana and Illinois. In stage 1, we observed apatite grains with reset fission tracks, and by modeling track length distribution, obtained a 184±13 Ma Jurassic rapid cooling age, consistent with the Mississippian-Pleistocene stratigraphic constraints on the age of the crater. We developed the working hypothesis that if our reset fission-track ages obtained from within the crater are related to impact, uplift, and exhumation, additional samples from outside the crater should yield fission-track ages that decrease to some background age within a few crater diameters away. The stage 2 subsurface samples, several crater diameter distances away, gave statistically identical cooling ages, failing our test. All but one stage 3 sample, tens of crater diameter distances from the crater center, gave pooled fission track ages statistically identical to the earlier, closer samples, again failing our test. We conclude that we did not date the age of the Kentland crater, but rather some regional-scale cooling and exhumation event that could either predate or postdate impact. This study illustrates some limitations related to the use of fission tracks to date deeply eroded craters.

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