Electronic Supplement to
Major Earthquakes Recorded by Speleothems in Midwestern U.S. Caves
by Samuel V. Panno, Craig C. Lundstrom, Keith C. Hackley, B. Brandon Curry, Bruce W. Fouke, and Zaofeng Zhang
Introduction
These supporting online materials contain a detailed description of sample collection methods, techniques used for petrography and growth-laminae counting of thin sections, uranium-thorium age-dating techniques, and a description of the geology and hydrogeology of the study area. In addition, we have included two data tables and five figures. Table 1 provides a comparison of stalagmite initiation dates with dates of known paleoearthquakes determined by other means. Tables 2, 3 provide the analytical results of uranium-series dating for all of the stalagmites dated in this investigation. Six figures provide photographs of stalagmites as they appeared in place and after being cored or collected and cut in half (including sampling locations for uranium-series dating). They also show thin sections in plain light and under cathodoluminescence in order to illustrate growth-laminae counting methods and the correlation of uranium-series dates with growth laminae counts.
Materials and Methods
Sample Collection
We collected, with permission from the Illinois Nature Conservancy and from the Department of Natural Resources, in accordance with the Illinois Cave Protection Act, and over the course of seven years, 15 stalagmites from Fogelpole Cave, Illinois Caverns and Pautler Cave (Figs. 1, 2). Because at least the upper part of the stalagmites were young and actively growing, both growth laminae counting in thin sections and standard U-Th disequilibria dating techniques (described below) were used to date the speleothems.
Petrography and Growth Laminae Counting
Thin sections were made from the nine white stalagmites using standard techniques. Each stalagmite was cut in half vertically along its growth axis and thin sections were made from one of the halves of each stalagmite. Thin sections were prepared by Burnham Petrographics, Rathdrum, ID. Thin sections were made with slightly thicker than normal and polished. The thin sections were initially examined under both transmitted and reflected light using a petrographic microscope in order to identify the character and fabric of the growth laminae.
Stalagmite growth laminae were counted in accordance with principles and guidelines proposed by Tan et al. (2006). Laminae were often obscured by other features (e.g., dissolution) or not identifiable along a single transects; to compensate for this, laminae along the sides of the stalagmites were counted and extrapolated to the transect being counted. Thin sections of each stalagmite were examined with a petrographic microscope under plain light (PL) and cathodoluminescence (CL). Uranium/Thorium ages were used to corroborate ages determined by counting growth laminae. Thin sections of several stalagmites were examined in detail. A transect of a thin section taken from the flank of stalagmite FC 94-1 was examined using plain light and CL (Figs. 3, 4). Lineaments within the thin section that we interpreted as growth laminae were created by the alignment of thin crystal faces lying normal to laminae boundaries, and by dissolution features. The crystal faces appeared (in plain light) as thin, dark, spiky features that highlight the boundaries; under CL, they appeared lighter-colored suggesting a more organic-rich content (Fig. 5). In other stalagmites, the crystal faces were often wider with thinner, clear calcite bands separating them. Contacts between laminae were usually sharp and often associated with parallel fractures that probably formed during the making of the thin sections. Along the FC 94-1 transect, the thickness of laminae ranged from 0.045 mm to 0.080 mm, with a mean, standard deviation, and median thickness of 0.055 mm, 0.01 mm, and 0.053 mm. The thickness of laminae along the flanks of the stalagmites is thinner, by almost an order of magnitude, than those along the central axes of the stalagmites. Eighty-three growth bands were estimated to be present between areas dated using U/Th dating techniques (65 ± 2 and 144 ± 5 calendar years). The total number of laminae between the two dated locations was based on the thickness of 32 identifiable laminae; the counts were within analytical error of the U/Th ages. Using two standard deviation about the mean, the growth laminae between the two U/Th ages yielded a time period between the samples of 81 ± 30 years.
In order to examine the entirety of each stalagmite, we used reflected light with the thin sections lying on a black background to bring out the growth laminae. Thin sections were scanned on an Epson flatbed scanner, and growth laminae were counted on 28 X 43 cm blow ups of the thin section scans. Because growth laminae were not visible everywhere on the thin sections (due to localized dissolution and recrystallization), clusters of continuous growth laminae were counted at multiple locations within each stalagmite thin section yielding an average number of growth laminae per mm thickness for each stalagmite. Laminae were most easily identifiable near the sides of the stalagmites where they could be counted and the upper and lower bounds of the laminae could be traced to the axes of the stalagmites. Assuming a linear growth rate, that value was multiplied by the height of the stalagmite to get the approximate age BP of the stalagmite. Counting error was estimated to be ± 18% based on the variation in thickness of annual growth laminae.
Uranium-Thorium Disequilibrium Age Dating
Speleothems were dated using standard U-Th disequilibria dating techniques (described below). Stalagmites were microdrilled in order to collect 100 mg samples along annual growth laminae. For most of the samples, the age of stalagmite concentric growth layers (laminae) were determined using a total dissolution single sample 238U-234U-230Th dating technique with measurements made on a Nu Plasma MC-ICPMS located in the Department of Geology at the University of Illinois. Analysis protocols are given in Sims et al. 2008. Chemical separation procedures for U-series analyses were similar to those of Edwards et al. (1986) and Cheng et al. (1998). MC-ICPMS analysis consisted of aspirating a 10 ppb solution of purified U or Th through a desolvating nebuliser and measuring either 238U, 235U, 236U and 234U or 232Th, 230Th and 229Th simultaneously on the mass spectrometer. Sensitivity for U and Th are routinely 400V/ppm at a nebulisation rate of 100 microliter/min. The instrument is fitted with a retarding potential lens providing abundance sensitivity of < 200ppb and an electron multiplier having a dead time of ~10 ns and dark current of < 6 cpm. Because of the young age of these samples, detrital Th corrections to the age are significant, but generally less than 10% (Tables 2). For these samples, a standard correction for the presence of detrital thorium was used, assuming the 230Th measured came from the detritus for some smaples, then the standard method for correction was no longer usable and the determined age can be meaningless. For example, sample IC-12 had a very large detrital Th content and yielded a U/Th age of 400 years ± 900 years. Consequently, the error can greatly exceed the age, rendering the age unusable. For some samples rich in detritus (Table 2), we performed an isochron analysis method whereby we drilled to spatially close samples having visibly different amounts of detritus. Each sample was completely dissolved and then measured for 238U-234U-230Th-232Th and a 2 point isocron age calculated.
Because any incorporation of the older, underlying carbonate minerals of the bedrock and older speleothems could significantly bias the age of the white stalagmites, particular care was taken to maintain a buffer zone between the oldest sample from the white stalagmite and the underlying older carbonate minerals. In addition, because of the presence of dissolution pits near the apex of the stalagmites, U-Th disequilibria ages were determined along unaffected growth laminae along the flanks of the stalagmites. The growth laminae sampled were then followed up to the apex of the stalagmite and the total age was extrapolated by measuring the total thickness of the stalagmite. This technique was cross checked using multiple U/Th ages of stalagmite FC94-1.
Geology and Hydrogeology of the Study Area
The study area includes three caves in the sinkhole plain of southwestern Illinois, Fogelpole Cave, Illinois Caverns and Pautler Cave. The sinkhole plain occupies parts of three counties and contains approximately 10,000 mapped sinkholes (with as many as 90 per km2), numerous large springs, and the longest caves in Illinois (Weibel and Panno, 1997; Panno et al., 2004a, b, c). Climatic conditions in the area are temperate and the area receives an average rate of precipitation of between 102 and 107 cm per year (Spatial Climate Analysis Service 2000). Here, Mississippian-age St. Louis Limestone is exposed along the western margin of the Illinois Basin and gently dips eastward toward the center of the basin. This relatively soluble, calcite-rich formation has prominent bedding planes and is highly jointed. Bedrock is overlain by a Quaternary succession of loess and glacial till that is replete with cover-collapse sinkholes. These deposits are as much as 20 m thick, and includes weathered pre-Illinoian silt and residuum (Oak Formation), Illinois Episode diamicton and silt (Petersburg Silt and Glasford Formation) capped by Wisconsin Episode loess (Peoria and Roxana silts) (Panno et al., 2004a). The bottoms of many sinkholes revealed bedrock. The uneroded Quaternary deposits are about 10 m thick in the vicinity of Fogelpole Cave, Illinois Caverns and Pautler Cave (Panno et al., 1996).
Fogelpole Cave, Illinois Caverns and Pautler Cave are branchwork-type caves (Panno et al., 2004a, b) with over 22 km, 10 km and 10 km of passages, respectively. All three caves have perennial first- and second-order streams that flow the lengths of their passages. These subterranean streams eventually discharge from relatively large springs down gradient of the distal end of the cave passages. Located within 4.5 km of each other, Fogelpole Cave and Illinois Caverns trend from northwest to southeast, and act as drains for their respective groundwater basins. Recent work by the authors has shown the caves to have formed about 150,000 years BP due to glacial melting near the end of the Illinois Glacial Episode (Panno et al. 2004b). Pautler Cave is located about 12 km northwest of Illinois Caverns; however, little additional information about the cave is available.
Figures Click an image to view full-size and/or view all images as a slideshow.
Fig. 1. A gallery of white stalactites and stalagmites in Illinois Caverns, many of which are growing on older, darker stalagmites.
Fig. 2. A photo montage of all stalagmites (in cross section) collected and analyzed during this investigation. IC = Illinois Caverns and FC = Fogelpole Cave. All scale bars are 1 cm wide.
Fig. 3. A photo montage of all stalagmites (in the field) cored or collected and analyzed during this investigation. IC = Illinois Caverns and PC = Pautler Cave.
Fig. 4. Projection of U-Th ages from the flank of stalagmite FC-3 to its apex and its use in determining the age of initiation of the stalagmites. Note that part of the top of the stalagmite was lost during thin section polishing.
Fig. 5. Transect across the flank of stalagmite FC 94-1 in plain light (left) and CL (right) showing the locations of U/Th age dated layers, and annual growth laminae.
Fig. 6. Stalagmite FC-9 in PL (left) and CL (right) showing boundaries between growth laminae as termination of crystal faces. Cathodoluminescence shows the crystal faces to be lighter in color suggesting a greater organic content. A scalenahedron of calcite is seen cross-cutting annual laminae in PL.
Table 1.
| Age (Years AD or BC) | Time BP (Years) | Source(s) | Evidence |
|---|---|---|---|
| AD 1917 |
100 ± 18* 88 ± 16* 84 ± 15* 83 ± 8* |
Panno et al. (this study) Panno et al. (this study) Panno et al. (this study) Panno et al. (this study) |
Stalagmite Initiation Stalagmite Initiation Stalagmite Initiation Stalagmite Initiation |
| AD 1811-12 |
200 ± 36* 200 ± 36* 191 ± 34* 206 ± 37* 198 ± 36* 196 ± 63 240 ± 56 210 ± 75 |
Nuttli (1987) Saucier (1989) Panno et al. (this study) Panno et al. (this study) Panno et al. (this study) Panno et al. (this study) Panno et al. (this study) Panno et al. (this study) Panno et al. (this study) Panno et al. (this study) |
Historic Records Liquefaction Stalagmite Regrowth Stalagmite Initiation Stalagmite Initiation Stalagmite Initiation Stalagmite Initiation Stalagmite Initiation Stalagmite Initiation Stalagmite Initiation |
| AD 1450 |
550 ± 150 620 ± 110 |
Tuttle et al. (1999a, 2002) Panno et al. (this study) |
Liquefaction Stalagmite Initiation |
| AD 900 |
1100 ± 100 1200 ± 260 |
Tuttle et al. (1999a, 2002) Panno et al. (this study) |
Liquefaction Stalagmite Initiation |
| AD 300 | 1700 ± 200 | Tuttle et al. (2005) | Liquefaction |
| 1620 BC |
3626 ± 220 3500 ± 100 ~ 3900 3050 ± 355 |
Holbrook et al. (2006) Dennison et al. (2000) Dennison et al. (2007) Panno et al. (this study) |
Miss. River Meander Stalagmite Initiation Stalagmite Initiation Stalagmite Initiation Stalagmite Initiation |
|
2244 BC 2350 BC |
4250 ± 269 4350 ± 200 4600 ± 130 |
Holbrook et al. (2006) Tuttle et al. (2006) Panno et al. (this study) |
Miss. River Meander Liquefaction Stalagmite Initiation |
| 3500 BC |
~ 5500 5460 ± 224 5700 ± 1800 |
Tuttle et al. (2006) Lepley (2004) Panno et al. (this study) |
Liquefaction Stalagmite Initiation Stalagmite Initiation |
| 4520 BC | 6502 ± 160 | Tuttle et al. (2004) | Liquefaction |
|
11,340 BC 10,000 BC |
13,340 - 9,000 12,000 ± 1000 10,190 ± 420 11,500 ± 70** |
Vaughn (1994) Munson et al. (1997) and Tuttle et al. (1999b) Lepley (2004) Panno et al. (this study) |
Liquefaction Liquefaction Stalagmite Regrowth Stalagmite Initiation |
| 21,000-15,000 BC |
23,000 - 17,000 17,780 ± 120 |
Vaughn (1994) Panno et al. (this study) |
Liquefaction Stalagmite Initiation |
*Based on growth laminae counting; stalagmites collected in 2006.
**Ages extrapolated to the bottom of the stalagmite.
Table 2. Data used in uranium-series dating of all stalagmites
| Sample | U (ppm) | Th (ppb) | δ234Ui | δ234Up | (230Th/238U) A | Uncorrected ages (ka) | Corrected ages (ka) |
|---|---|---|---|---|---|---|---|
| FC94-1-high | 12.62±0.02 | 4.984±0.008 | -164.0±0.4 | -163.9±0.4 | 0.0006033±0.0000060 | 0.079±0.001 | 0.065±0.002 |
| FC94-1-low | 11.70±0.02 | 11.79±0.02 | -155.9±0.4 | -155.8±0.4 | 0.001392±0.000012 | 0.179±0.002 | 0.144±0.005 |
| FC-3 | 4.950±0.010 | 6.671±0.009 | -47.15±0.11 | -47.14±0.11 | 0.001178±0.000017 | 0.135±0.002 | 0.093±0.005 |
| FC94-1A | 8.902±0.026 | 162.5±0.53 | -156.7±0.4 | -153.9±0.3 | 0.03944±0.00044 | 5.20±0.07 | 4.56±0.10 |
| FC-3A | 14.24±0.11 | 54.27±1.09 | -164.0±0.4 | -161.6±0.4 | 0.03571±0.00078 | 4.74±0.13 | 4.60±0.13 |
| FC-1A | 6.614±0.015 | 12.20±0.03 | -127.6±0.3 | -123.9±0.3 | 0.07972±0.00038 | 10.40±0.07 | 10.33±0.06 |
| FC-1B | 10.68±0.03 | 8.338±0.019 | -152.2±0.3 | -147.4±0.3 | 0.08291±0.00040 | 11.16±0.07 | 11.13±0.07 |
| FC-1C | 10.49±0.03 | 19.91±0.04 | -125.6±0.3 | -119.4±0.3 | 0.1328±0.0006 | 17.85±0.12 | 17.78±0.12 |
| FC-1D | 11.24±0.03 | 6.813±0.015 | -174.9±0.4 | -174.9±0.4 | 0.0007413±0.0000224 | 0.098±0.003 | 0.076±0.004 |
| IC1 | 1.742±0.004 | 14.62±0.03 | 411.0±0.9 | 410.6±0.9 | 0.002253±0.000084 | 0.174±0.013 | 0.009±0.017 |
| IC5 | 4.398±0.010 | 22.52±0.05 | 137.1±0.3 | 137.0±0.3 | 0.002398±0.000065 | 0.230±0.006 | 0.096±0.017 |
| FC9 | 4.167±0.009 | 4.250±0.009 | -136.2±0.3 | -136.2±0.3 | 0.001491±0.000051 | 0.188±0.006 | 0.153±0.008 |
| IC10 | 1.333±0.003 | 35.37±0.08 | 173.8±0.4 | 173.5±0.4 | 0.006551±0.000093 | 0.609±0.009 | 0.018±0.037 |
| IC12 | 0.7096±0.0016 | 53.40±1.19 | 549.3±1.2 | 529.1±1.2 | 0.1761±0.0009 | 13.22±0.08 | 0.4±0.9 |
Table 3. Data used in isochron uranium-series dating of stalagmites.
| Sample | Isochron ages (ka,2σ) | Detritus (230Th/232Th) A | Sample Point Used |
|---|---|---|---|
| IC-22 | 240 ± 56 | 1.073 | 2 |
| PC-1 | 5,700 ± 1,800 | 0.02 | 2 |
| PC-2a | 990 ± 260 | 0.76 | 2 |
| PC-2b | 3,050 ± 355 | 0.68 | 2 |
| PC-3 | 620 ± 110 | 1.835 | 2 |
| PC-6 | 196 ± 163 | 0.49 | 2 |
| PC-7 | 4,300 ± 210 | 0.67 | 2 |
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