A Re-interpretation of Pennsylvanian "Filled-sinks" in Missouri
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For a century and a half, isolated sandstone bodies scattered in a belt across the central part of Missouri have been interpreted as Pennsylvanian age filled sink hole deposits, based on their geometry, distribution, and that they appeared to be the first significant sandstone above the Ordovician carbonate units. However sedimentary facies and facies associations indicate that these sandstones were deposited by running water, possibly as valley-fills over paleokarst surfaces. Additionally, the units are often overlain by Ordovician through Mississippian strata and residuum, suggesting they range in age. The purpose of this study is to map these bodies using modern GIS techniques and to more fully document and describe their physical characteristics in order to better understand their origin and timing.
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A Re-interpretation of Pennsylvanian "Filled-sinks" in Missouri
- 1. A REEXAMINATION OF PENNSYLVANIAN “FILLED-SINK” DEPOSITS IN EAST-CENTRAL MISSOURI: ARE THEY PENNSYLVANIAN AND ARE THEY FILLED SINKS? William W. Little Missouri Department of Natural Resources Geological Survey and Resource Assessment Division
- 3. Small circular outcrops Dickite Disturbed bedding Stratigraphic position Evidence Supporting Pennsylvanian Sink and Cave Fill Features
- 4. Oc Oc “Psf”
- 8. A B C D B D
- 11. Mc Oc
- 12. Mc Mb? Oc
- 14. “Psf” (rimrock) Remnant paleokarst hills Oc
- 15. “Psf” (rimrock) Remnant paleokarst hills Oc
- 17. Oc Mb Mc
- 18. ds Interbedded ss, ms, & ds} Paleokarst Hill on Oc
- 19. ds Interbedded ss, ms, & ds} Paleokarst Hill on Oc
- 20. ds
- 21. ds
- 22. ds ss & ms
- 23. Middle O Everton Fm on I-55 “P sink-fill” on Hwy 50
- 24. Outcrop pattern and measured sections for Middle Ordovician strata Generalized Section Measured Sections Cave Hill Rd Exposure
- 29. Versailles Mr “Pss” Goerlisch Ridge Mr “Pss” Vichy Mr “Pss” Otterville East Mr “Psf” Oc Morrison Mr “Pss” Fredricksburg Mr “Pss” New Melle Mr “Pss” Washington East Mr “Pss”
- 30. “Pss” Mr “Pms”
- 31. Mississippian Residuum from atop a Pennsylvanian sandstone Modern stream gravel from same area
- 32. Mississippian Residuum from atop a Pennsylvanian sandstone Glacial erratics from the Troy area
- 39. South side of hwy North side of hwy Differential Compaction
- 40. Oc “Pss” Oc
- 41. • In some places, these deposits are interbedded with carbonates of Ordovician age. • In other localities, they underlie Mississippian-age limestones. • As shown on the state geologic map, small remnants of Devonian and Mississippian-age strata locally overlie these deposits. • Cobbles and small boulders of fossiliferous Mississippian-age chert are found on the surface above these deposits throughout their mapped extent. Evidence Supporting a Pre-Pennsylvanian Age
- 42. • Most deposits are flat-lying with disturbed strata being restricted to lateral margins. • The regional distribution pattern of sandstone resembles that of an integrated drainage system and commonly forms a sheet. • Internal structures of sandstones indicate deposition by moving water. • Sandstone bodies are often flanked by stratified successions of interbedded claystone, siltstone, and thinner sandstones. Evidence Supporting a Paleovalley Origin
Editor's Notes
- Oral Presentation to the Geological Society of America North-Central Section Annual Meeting, 1 April 2004. Annotations added October 2012.
- Red rectangles show areas mapped by the Missouri Geological Survey between 1998 and 2004 that include features interpreted as Pennsylvanian age “filled sinks.” These areas, from west to east, include Truman Lake near the town of Warsaw, the Lake of the Ozarks region, and the area along the Missouri River between Fulton and St. Louis. The larger, black rectangle corresponds approximately to the area in which “filled sinks” have been identified.
- For over 100 years, some sandstones and mudstones in these regions have been interpreted as Pennsylvanian-age sink and cave fills, primarily because of a roughly circular outline for many of the clay pits, the presence of high temperature clay minerals, bedding which is sometimes chaotic, and stratigraphic position, as the first significant sandstone above known Ordovician carbonate units.
- My interest in these deposits began in 1998 while mapping outcrops along the shoreline of Harry S. Truman Reservoir, where I encountered this “rubble zone,” which I was instructed was a “Pennsylvanian filled-sink” deposit. It consists of a gravelly chert “rubble zone” bounded sharply on both sides by the Ordovician Cotter Dolomite (Oc) and capped by a poorly-cemented Pennsylvanian “rimrock” sandstone. I was informed that this was typical for “Pennsylvanian filled-sink (IPsf)” deposits.
- This is the same outcrop as in the previous slide, but from another angle. Note the exposure of the Mississippian Compton Limestone (Mc) in the distance.
- White rectangle shows the area enlarged in the next slide.
- Tracing the “IPsf rimrock” laterally, we found that it was overlain by the deposit of Mc shown in the previous slide. Some colleagues suggested this to represent a cavity within the Oc but below the Mc that had been filled by a “IPsf.”
- Facies of the Truman Lake “filled-sink” deposit. A) Upper part of deposit composed of gravelly trough cross-bedded sandstone (orange) overlain by trough cross-bedded sandstone without gravel (gray). B) Close-up of gravelly sandstone (labeled in Photo A). C) Chert-pebble conglomerate with sandstone matrix at base of outcrop (below Photo A). D) Ripple-bedded sandstone at top of outcrop (labeled in Photo A). I thought this outcrop more resembled a fining-upward channel fill than a chaotic collapse deposit. Together with the overlying Mississippian carbonates, I became skeptical of the traditional interpretation and began to examine other exposures of the contact between Ordovician and Mississippian deposits in the Truman Lake area.
- In the Truman Lake region, the contact between the Cotter Dolomite (Oc) and Compton Formation (Mc) is an unconformable karst surface spanning more than 100 ma. The character of this contact is highly variable. In places, the two formations are in direct contact; elsewhere, they are separated by either a thin green mudstone or by sandstones, such as the Bachelor Formation (Mb) and sandstones interpreted as “IP filled sinks,” as shown in previous slides, demonstrating a complex sea-level history with a varied expression during this hiatus. This photo is of the west side of Highway 83 immediately south of the Pomme de Terre branch of the lake.
- Boxes show localities of close ups in the next slides.
- Close up of Box A in the previous slide, showing separation of the Oc and Mc by a green shale a few centimeters thick.
- Close up of Box B of the earlier slide, showing separation of the Oc and Mc by an indurated quartz arenite sandstone resembling the Mb.
- Small outcrop of Mb on Truman Dam Access Rd, about 0.5 mi west of the intersection with Hwy 65. Here, the Mb consists of a sandy chert-pebble conglomerate.
- Outcrop of “IPsf” rimrock overlying Oc. Located on Truman Dam Access Rd, about a couple hundred feet east of previous slide. “IPsf” consists of fine-grained, trough cross-bedded quartz arenite. The contact between “IPsf” and Oc has considerable relief with nearly vertical walls, probably representing solution valleys into Oc.
- The rectangle shows the area enlarged in the next slide.
- Enlargement of area marked by a rectangle in the previous slide. This is a very fine-grained, well-sorted, friable quartz arenite with distinctive trough cross-bedding. What appear to be gravelly regions are mostly concretions or zones of higher cementation.
- Small outcrop of Mb on the north side of Hwy 7 at the west end of the bridge across the Osage Branch. Consists of a sandy chert-pebble conglomerate between Oc below and Mc above.
- North side of Hwy 50 just east of the intersection with Cave Hill Rd. This outcrop has been interpreted as a “IPsf” and consists of interbedded sandstone (mostly quartz arenite), mudstone, and dolostone unconformably overlying the Oc, with a contact that suggests paleokarst.
- The rectangle shows the area enlarged in the next slide.
- Close up of area in rectangle of previous slide.
- The rectangle shows the area enlarged in the next slide.
- Close up of area in rectangle of previous slide.
- Comparison of two very similar outcrops. The upper photo represents a “IPsf” on the south side of Hwy 50 (across the highway from earlier slide). The lower photo is of the Middle Ordovician Everton Formation found on I-55. I believe that both outcrops are of Ordovician age.
- Map showing the location of the Cave Hill Road and Hwy 50 exposures of “IPsf” to very similar units of accepted Middle Ordovician age.
- At the time of my talk, this was interpreted as a “IPsf;” however, the completed geologic map now shows this as Ordovician St. Peter Sandstone (Os) but does not illustrate the interbedded nature with Jefferson City Dolomite (Oj). I believe that many of the outcrops identified as “IPsf” are, in actuality, Os. This outcrop is located along the east bank of Boeuf Creek at Camp Trinity, approximately 2 miles southwest of the town of New Haven. The interbedded character of sandstone and dolostone is similar to that shown earlier along Hwy 50 near Cave Hill Road.
- A common occurrence through much of central Missouri is to find angular chert residuum containing Mississippian age fossils (Mr) overlying exposures of “IPsf.” In some instances, the Mr consists of scattered fragments. In others, such as this photo, it can form a thick blanket. This has been explained as both glacial till and colluvium. However, many exposures are much too far south to be related to glaciation, and colluvium should form a thin ribbon along the margins of an ancient stream; whereas, these deposits are ubiquitous, forming a sheet across much of central Missouri. This photograph was taken on the north side of Hwy F, about a half mile west of the intersection with Hwy W, approximately 6 miles east of the town of Pershing.
- The rectangle shows the area enlarged in the next slide.
- Close up of area in rectangle of previous slide.
- Other localities across the state with Mr overlying “IPss.”
- We visited this outcrop as part of a field review before publishing the 2004 maps. I again brought up the stratigraphic problem of having Mississippian-age rocks overlying Pennsylvanian-age rocks. The response of one of my colleagues was: “These have been called Pennsylvanian sink fills for 150 years; who are we to change it?” I think that herein lies the difficulty, fear to break with convention, even when evidence to the contrary is overwhelming. These were described as Pennsylvanian filled sinks at a time when access and data were limited. We also have new techniques, such as sequence stratigraphy, to explain things we couldn’t in the past. We now have a team of mappers that covers 12 to 16 7.5’ quadrangles per year and roads that take us to places that were hard to get to in the past. We are the mappers; we have much greater access; we have much more data; it is our job to modify, where warranted, earlier interpretations.
- Because Mr resembles modern stream gravel of the same material, some colleagues have suggested that it represents a layer of colluvium deposited on top of the “IPsf” deposits. However, colluvium, if preservable, would more likely be present as ribbons of gravel along the margins of fluvial channel deposits; whereas, this material forms a blanket that covers much of the central part of the state of Missouri.
- Others consider Mr to be the result of Pleistocene continental glaciation; however, compositionally and texturally, the blanket of Mr is considerable different than known glacial deposits, and they are found considerable further south than the accepted limits of Pleistocene glaciation.
- Even the official state geological map shows outcrops of Devonian and Mississippian strata overlying Pennsylvanian rocks. In this region, the pattern produced by “Pennsylvanian sandstone” is reminiscent of an inverted paleodrainage pattern.
- The previous discussion dealt mostly with age. What about it’s origin? If not filled sink deposits, what are they? This photograph is part of the Cave Hill Road exposures shown in Slide 24. The historical interpretation of this outcrop is a “IPsf” deposit with “rimrock” dropped down within Oc.
- Boxes show localities of close ups in the next slides.
- Close up of area in box A of previous slide. The “IPsf (rimrock)” is a lens-shaped sandstone dominated by trough cross-stratification, reminiscent of an active fluvial channel deposit.
- Close up of area in box B of Slide 35. The “IPsf” deposit consists of interbedded sandstones and mudstones. Some of the sandstones contain small-scale cross-stratification and the mudstones have variegated colors, suggestive of paleosols. My interpretation is overbank and splay deposits.
- Similar exposure on a county road a little north of Rolla.
- One of the cited evidences for sink collapse is the deformed margin that surrounds some of the “rimrock” deposits. This exposure along Highway 50, between the city of Sedalia and the Lamine River, shows a thick “IPsf (rimrock)” on the south side of the highway that disappears and is represented by only a small synclinal fold in carbonate rocks on the north side. This is most likely due to the lower compressibility of sandstone with respect to carbonate mud and could why explain rock bodies that dip toward “rimrock” sandstones.
- This roadcut, also along Hwy 50 and near to the locality of the previous slide, shows a “rimrock” sandstone bounded by faults. Again, I believe due to differential compaction.