Petrology of Pliocene Mississippi River Alluvium ...

Petrology of Pliocene Mississippi River Alluvium: Provenance Implications David N. Lumsden,* Randel T. Cox, Roy B. Van Arsdale, and William B. Cupples Department of Earth Sciences, University of Memphis, Memphis, Tennessee 38152, USA

ABSTRACT Pliocene Mississippi River terrace gravels, the Upland Complex, crop out east and west of the present Mississippi River in the northern Mississippi Embayment. As the only sedimentary unit in the northern Mississippi Embayment deposited between the end of the Eocene and the onset of glaciation, its origin provides ground truth about conditions that existed in the heartland of North America within this 30-m.yr. Interval. Recent studies concluded that the Pliocene Mississippi River originated in what is now southern Canada and that the Upland Complex is the remnant of a much larger deposit that once extended from Canada to the Gulf of Mexico. Is there some evidence in the petrology of the Upland Complex that can confirm this conclusion? To determine the provenance of the Upland Complex we sampled 18 exposures, analyzed ∼50 thin sections, and obtained source terrane age data from its zircons. Upland Complex gravel came from proximal early Paleozoic carbonate rocks containing bedded and nodular chert north and northeast of the Mississippi Embayment with a possible distant contribution from as far away as southcentral Canada. Upland Complex sand came from multiple sources, including the Saint Francois Mountains of Missouri, the Grenville Terrane, and, possibly, southcentral Canada. We conclude that the Pliocene Mississippi River probably drained a much larger area than present Mississippi, an area that extended well north of the US-Canada border into Manitoba and Ontario.

Introduction [T]he Saskatchewan River which rises in the Rocky Mountains beyond our [the United States] northern boundary, was formerly the upper course of the Mississippi. (Gilbert 1877, p. 126)

may have been six to eight times that of the present Mississippi River (Cox et al. 2014; Cupples and Van Arsdale 2014). Stratigraphic evidence in support of these proposed Canadian headwaters may be in the petrology of the Pliocene Upland Complex (UC) gravel and sand of the northern Mississippi Embayment. Is there something in the lithology of the UC that can be used to support or contradict the extension of PMR drainage into southern Canada? To answer this question we undertook this study, an analysis of UC gravel and sand in a search for characteristics that can be used to identify its provenance. The northern Mississippi Embayment is a southplunging erosional trough containing a maximum

The current midcontinent North America drainage divide is roughly the US-Canada border, ∼497 N; the modern Mississippi River system drains south, and Canadian rivers drain north. However, the Pliocene drainage divide is uncertain. Galloway et al. (2013) placed it at ∼437300N, the latitude of the IowaMinnesota border. Cox et al. (2014) proposed a divide much farther north, ∼547N, the latitude of James Bay, Canada (fig. 1), on the basis of studies that concluded that preglacial rivers in southern Canada flowed southeast (Pugin et al. 2014) and that the discharge of the Pliocene Mississippi River (PMR)

of ∼1.5 km of Late Cretaceous and Paleogene sediments. The trough eroded when the Mississippi Valley graben was uplifted as it passed westward over the Bermuda hotspot in the mid-Cretaceous (Cox and Van Arsdale 1997, 2002). The Late Cretaceous Zuni Sea invaded the embayment as it cooled and subsided and deposited marine sediments. During the Paleogene period, the embayment filled from the

Manuscript received June 25, 2015; accepted April 5, 2016; electronically published June 21, 2016. * Author for correspondence; e-mail: [email protected].

[The Journal of Geology, 2016, volume 124, p. 501–517] q 2016 by The University of Chicago. All rights reserved. 0022-1376/2016/12404-0005$15.00. DOI: 10.1086/686997

501

502

D . N. L U M S D E N E T A L.

Figure 1. Map of North America illustrating our hypothesized Pliocene Mississippi River (PMR) drainage divide. Silurian to Mississippian, Proterozoic, and Archean bedrock exposures in the United States and Canada are modified after Cook and Bally (1975) and Reed et al. (2005). Upland Complex distribution is modified after Saucier and Snead (1989) and Cupples and Van Arsdale (2014). Black dashed lines extend the purported course of the modern Mississippi River and tributaries into southwestern Ontario and adjacent Manitoba (Cupples and Van Arsdale 2014). The Hatfield and Spiritwood regional network of subtill preglacial valleys mapped by Pugin et al. (2014) is shown as connected to the modern Mississippi River drainage. A much generalized trend for the Oligocene Bell River is indicated by parallel dashed lines (Sears 2013). SA p Severn Arch. See text for details.

north with Paleocene and Eocene fluvial and deltaic deposits; by the end of Eocene, the embayment was full (Lumsden et al. 2009). Subsequent Oligocene to middle Miocene sediments bypassed the northern Mississippi Embayment to form deltaic deposits along

the Gulf Coast (Galloway et al. 2013). After a middle Miocene interval of sizable deposition, sediment influx into the gulf was low during the late Miocene and Pliocene, thereafter increasing to a maximum during the Pleistocene (Galloway et al. 2013). The

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P L I O C E N E M I S S I S S I P P I RI V E R

low during the Pliocene coincides with apparent sediment storage in the embayment that we interpret as the UC. The UC (aka Lafayette Gravel, Mounds Gravel, and Citronelle Formation) is interpreted to be the remnant of a deposit of the PMR that once formed a continuous blanket up to 100 m thick, ≥100 km eastwest (from the Chickasaw Bluffs in western Tennessee west to at least Crowley’s Ridge in eastern Arkansas; fig. 2), and 700 km north-south (along the Mississippi River from Iowa to southern Louisiana; Potter 1955a, 1955b; Saucier and Snead 1989; Autin et al. 1991; Van Arsdale et al. 2007; Dockery et al. 2008; Lumsden et al. 2010, 2012, 2015; Cox et al. 2014; Cupples and Van Arsdale 2014). Cupples and Van Arsdale (2014) mapped discontinuous UC (Mounds Gravel) on ridgetops extending along the Mississippi River to northernmost Illinois. The Citronelle Formation of the Gulf Coast is the southern extension of the UC (Otvos 1988, 2004). Sea level decline to 220 m during the early Pleistocene resulted in incision by the Mississippi River that isolated the UC as a terrace. The UC silt and sand facies has been eroded, leaving behind the remnant basal sand and gravel terrace that is the focus of this study (Van Arsdale et al. 2007). In the northern Mississippi Embayment, the UC disconformably overlies Eocene formations and is disconformably overlain by Wisconsinan loess (table 1). Stratigraphic relationships are consistent with a Pliocene age (Potter 1955a, 1955b; Willman and Frye 1970; Otvos 1988, 2004; Autin et al. 1991; Saucier 1994; Anthony and Granger 2006; Van Arsdale et al. 2007). There are no contemporaneous fossils in the UC except for common but undiagnostic petrified wood. Ophiomorpha tubes identify estuarine beds within the dynocist- and pollen-dated late Pliocene Gulf Coastal Citronelle alluvium (Otvos 1998). The Pliocene marine Perdido Key and Jackson Bluff Formations underlie and/or intertongue with the Citronelle (Huddlestun 1988; Otvos 1988, 1998, 2004). The Pliocene age of the UC was confirmed when quartz 26Al-10Be dates from a quarry in the Chickasaw Bluffs (location 9, table 2) gave burial ages of 3.1 5 0.5 and 3.35 5 0.3 Ma (Van Arsdale et al. 2014). Deposition of the UC coincides with the midPliocene warm period (MPWP, 3.3–2.9 Ma; Pagani et al. 2009; Winnick and Caves 2015; http://www .pliomax.org). The MPWP may represent global climatic conditions that our current environment may be trending toward. Therefore, conclusions presented here may prove to have implications related to climatic trends occurring today in midcontinent North America.

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Methods We sampled the UC at 18 exposures in Tennessee, Kentucky, Mississippi, Missouri, and Illinois, largely in gravel quarries, and visited many other locations (fig. 2; table 2). For comparison purposes, we also sampled the Tuscaloosa Gravel (Late Cretaceous) in Kentucky and Tennessee, the Pleistocene Grover Gravel in Missouri (Rovey 2012), and the Mounds Gravel in Illinois (Nelson et al. 2003; table 2). Semiconsolidated samples were impregnated with epoxy resin to facilitate standard thin-section preparation. Poorly consolidated samples were disaggregated, washed, and sieved to separate sand from the 12.0-mm and !0.062-mm fractions. Sand fraction grain size distribution was determined using routine sieve analysis (half f intervals). Sand grains were divided into monocrystalline quartz, polycrystalline quartz, chert, feldspar, and partial- to near-complete replacement of chert by goethite. Sand composition of four size fractions was estimated by grain counts of thin sections. X-ray diffraction was used to determine the composition of fines, !0.062 mm. Opaque areas in thin sections were analyzed using infrared reflectance microscopy (Parish et al. 2013). Approximately 100 two- to five-centimeter-diameter hand specimens of chert from several locations were partitioned into three groups—featureless clasts were assumed to be from bedded chert, and clasts with traces of fossils, oolites, or fine laminations were assumed to be nodular chert; a third chert group comprised those of uncertain attribution. Approximately 50 gravel-sized clasts (12.0 mm) were divided into bedded and nodular chert on the basis of texture in thin section. Heavy-mineral analysis followed Potter’s (1955a) procedure. The 0.5–0.088-mm fraction was separated from samples of the Missouri Grover Gravel, Arkansas Crowley’s Ridge UC, and the Tennessee Chickasaw Bluffs UC (locations 6, 9, and 23, table 2). The samples were leached with sodium dithionite to remove iron oxide coatings. Grains that settled in metatungstate were mounted on smear slides and counted using a petrographic microscope. Zircon ages were obtained for fine-sand samples from Arlington and Perry quarries in southwestern Tennessee and northwestern Mississippi (locations 4 and 9, table 2; R. Hatcher, personal communication, zircon age date data, 2015). Results The UC has been interpreted to be the basal sand and gravel facies of a thick alluvial floodplain (Van Arsdale et al. 2007; Cupples and Van Arsdale 2014);

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Figure 2. Location map (after Cox et al. 2014). Regional location is shown in the insert. The Upland Complex underlies the areas outlined in white (the Chickasaw Bluffs east of the Mississippi River and Crowley’s Ridge west of it). Approximate sample locations indicated by small white circles are precisely located in table 2. Rose histograms denote cross-bed dip directions. M p Memphis, Tennessee; N p New Madrid, Missouri.

however, sedimentary structures in the majority of exposures resemble those formed by deposition in a braided river (fig. 3). Braided rivers typically consist of a network of cross-bedded sand-dominated chan-

nels, transverse bars (T-bars), separated by longitudinal bar (L-bar) islands of massive bedded gravels, the latter deposited during high-velocity flood conditions (fig. 3A). As a consequence, given accom-

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Table 1. Relevant Stratigraphy Holocene Epoch Mississippi River alluvium Pleistocene Epoch loess deposits vary in area and age (Lumsden et al. 2003) Peoria Loess—top of the Peoria ∼10,000 yr BP Marianna Loess—base of Marianna ∼800,000 yr BP Disconformity—erosional contact ————————————————————————— Pliocene Epoch—from 2.58 to 5.33 Ma UPLAND COMPLEX 3.1 5 0.5 and 3.35 5 0.3 Ma Pliocene Mississippi River terrace ————————————————————————— Disconformity—erosional contact Eocene Epoch—from 33.9 to 56.0 Ma Claiborne Group Cockfield, Cook Mountain, Memphis Sand Formations (Lumsden et al. 2009)

modation space, braided rivers deposit an intricate association of gravels and coarse sands (fig. 3B). UC gravels resemble L-bar deposits (fig. 3C), whereas the associated cross-bedded coarse sands resemble T-bar deposits (fig. 3D). In some exposures, gravel and sand interbed in a complex pattern (fig. 3E). Exceptions to the braided dominance include point bar deposits within the UC in northern Crowley’s Ridge (Guccione et al. 1986) and in northwestern Mississippi (Cox et al. 2014; fig. 3F). Alluvial fan deposits occur in the UC of western Kentucky (Potter 1955b; fig. 3G). We estimate a minimum duration for deposition of the UC of ∼800,000 yr, on the basis of its estimated maximum 26Al-10Be age of 3.3 Ma to when entrenchment formed the terrace (first major continental glaciation, ∼2.5 Ma; Balco and Rovey 2010). The UC is dominated by strong brown to very pale brown (Munsell color 7.5 YR 4/6 to 10 YR 8/6) massive bedded well-rounded chert pebble gravel and coarse quartz sand. Gravel clasts are almost entirely chert with some polycrystalline quartzite (!5%). Chert clasts are stained with a surficial coating of goethite (FeO(OH)), 1.0–2.0 mm thick, that also extends along cracks (fig. 4A). The polished redbrown patina shows no percussion marks, an indication that the brown stain is postdepositional (fig. 4B). Smaller clasts (!2.0 cm) are frequently brown stained throughout or are partially to completely replaced by goethite (fig. 4C). Common unstained clasts vary in abundance without an observable geographic pattern. Chert clasts can be divided into three groups based on texture in thin sections, two bedded, one nodular. In one bedded-chert group, the clasts are composed of fine- to medium-grained microcrystalline quartz (fig. 4A), with many samples having ghosts of sponge spicules (fig. 4C). In the second beddedchert group, the finely crystalline chert contains dolomite rhombs, dolomolds, and chert and goethite pseudomorphs after dolomite (fig. 4D). This tex-

ture is common in Lower Mississippian carbonate units, such as the Fort Payne, Boone, and Burlington Formations in the present Mississippi River basin (Lumsden 1988, 2003). Nodular cherts, a third group, commonly preserve mid-Paleozoic fossils, some identifiable to the genus level (Dockery et al. 2008). Thin sections of putative nodular chert exhibit ghosts of intraclasts, pellets, oolites, and fossils (fig. 5). The ratio of bedded to nodular chert is 3∶2. Nodular chert that replaced micrite is easily confused with specimens of bedded chert, limiting the significance of hand specimen counts. Potentially provenance-diagnostic unusual components of cobble to boulder size are beautifully illustrated in Dockery et al. (2008). These include the erratic occurrence of unstained boulders of chertreplaced limestone up to 1.0 m long, mostly angular. We know of no systematic study of these boulders; however, cursory examination suggests a midPaleozoic strata provenance. Centimeter-scale geodes lined with quartz crystals (Keokuk geodes) are common but are not diagnostic of a source area (Hayes 1964). A provenance-specific component is Potosi agate (aka Missouri Lace agate), distinctive banded chert nodules from Cambrian dolostones in southeast Missouri. Clasts of Sioux Quartzite, although uncommon, suggest a contribution from Minnesota. Large (centimeter-scale) unrounded fossils of petrified wood are also common. We suspect that they originated as waterlogged wood and that they are contemporaneous with the UC. Cursory examination suggests similarity to modern trees (D. Burnette, personal communication, 2015). Sand textures include abundant sharply angular, limpid, deeply embayed grains of probable volcanic origin (fig. 6A). Optically continuous quartz overgrowths on dust rims of well-rounded monocrystalline quartz grains and grains of fine sandstone point to a sedimentary rock source (fig. 6B, 6C). Plentiful polycrystalline grains with complex sutured

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Table 2. ID

Investigated Exposures

Location

Name

Character

Latitude

Longitude

Comment Upland Complex—Chickasaw Bluffs, Kentucky, Tennessee, Mississippi 1

CB

2

CB

3

CB

4

CB

5

CB

6

CB

7 8

CB CB

9

CB

10

CB

11 12

CB CB

Coffeeville, MS, Fly MSG Batesville, MS, Brasell MSG Crenshaw, MS, MSG Senatobia, MS, Perry MSG Olive Branch Bridge MSG Hernando, MS, Andersen MSG West Byhalia, MS Byhalia, MS

Massive and X-beds Massive and X-beds Massive and X-beds Massive and X-beds Massive and X-beds Massive and X-beds ? ND

3375603900

28975405400

Southernmost

3471702400

2897580300

Eocene exposed

3472801200

29070804000

X-beds dip 507 SE

347390

29070502400

X-beds dip 307 NE

3475603000

28975302400

3475204900

28975701100

X-beds dip N

3475302500 3475403300

28974403900 28974002000

Arlington, TN, North MSG Old River, TN

Massive

3571800900

28974405000

Small exposure Standard Construction Co. Eocene exposed

Massive and X-beds Massive Alluvial fan

3572500300

28975802900

Top/bottom contacts

3871801300 3674900200

28972102400 28874001700

Eocene exposed Northernmost

Reelfoot, TN Old Hickory Clay Co., KY

Upland Complex—Crowley’s Ridge, Arkansas 13 14

CR CR

Beech Grove, AR Paragould, AR

15 16 17 18

CR CR CR CR

Jonesboro, AR Harrisburg, AR Levesque, AR Wittsburg, AR

Braided Braided meandering Braided ? Braided Braided

3670702000 3670102000

29073704800 29073401000

Large exposure Large exposure

3574505200 357320800 3571501700 3571102400

29074204500 29073903200 29074303000 29074100000

Recreation park Eocene exposed APAC quarry Large exposure

28774403800 28871405900 289.209721 288.827429 290.675525 290.631561

Whitten quadrangle Large exposure South Illinois South Illinois SE Missouri SE Missouri

Other locations 19 20 21 22 23 24

Tuscaloosa Tuscaloosa Mounds Mounds Grover Grover

Wayne Co., TN Livingston, KY Type section Long Road Type section Wildwood

Massive Massive Massive ? Massive ?

3570104500 3770105500 37.120535 37.227623 38.572343 38.581592

Note. Locations are in figure 1. CB pChickasaw Bluffs; CR pCrowley’s Ridge and other locations; MSG pMem- phis Stone and Gravel Company quarries; ND pnot determined.

contacts among crystallites indicate a metamorphic source (fig. 6D). Abundant angular and rounded, unaltered feldspar (both K-spar and plagioclase) in Crowley’s Ridge samples suggest a proximal granite pluton source (fig. 6D). Overlapping sources are suggested by the occurrence of sharply angular and embayed quartz, feldspar, and well-rounded monocrystalline and polycrystalline grains in the same thin section. We did not observe mudrocks, that is, overbank floodplain deposits; however, their former presence is suggested by abundant sand-sized kaolinite clasts (fig. 7A). We suspect that the clasts are tiny mudballs that formed by undercutting of flood plain over bank deposits. Soil-forming processes are sug-

gested by ubiquitous, red-stained, secondary kaolinite cement (fig. 7B). Patchy goethite cement preceded the kaolinite (fig. 5C). We analyzed four size fractions of s a n d fro m the Chickasaw Bluffs, Crowley’s Ridge, Tuscaloosa Gravel, Mounds Gravel, and Grover Gravel (table 3). It is clear from these data that UC sand composition differs profoundly from that of the Late Cretaceous Tuscaloosa. This is consistent with the hand specimen observation that Tuscaloosa gravel clasts seldom contain fossils (fig. 4B) and with the comparison of textures in thin section (compare fig. 2C–2E to fig. 3F). In sum, the Tuscaloosa was not a significant contributor to the UC. Data in table 3 confirm that the Grover Gravel and the UC are very different

Figure 3. Diagram and photographs of exposures. Location numbers correlate with table 2. A, Schematic diagram illustrating the idealized stratigraphic and geographic relations among transverse bar (T-bar) and longitudinal bar (Lbar) facies in a braided stream. B, Stack of four units capped by 3 m of a massive T-bar deposit (location 2). C, Crude coarsening-up sequence in a T-bar deposit (location 4). D, Epsilon cross-beds in an L-bar deposit (location 4). E, Intertonguing gravel and sand-dominated facies interpreted as due to braided deposition (location 14). F, Very large cross-bedded sand and gravel that could be a point bar or a T-bar deposit (location 6). G, Alluvial fan deposits characterize the Upland Complex in western Kentucky (location 12).

Figure 4. A, C–F, Thin-section photomicrographs taken with uncrossed polarizers. B, Photograph of hand specimens. A, Clast of very fine chert with surface coating of goethite that extends along cracks. B, Middle clast from the Upland Complex is coated with a thick red brown rind of iron oxide that has a high polish and no evidence of percussion marks (Munsell color 7.5 YR 4/4). Clasts to the right and left are from the Tuscaloosa Gravel (Munsell color 2.5 R 8/1). C, Sponge spicule chert clast partially replaced by goethite. D, Fine crystalline chert with abundant dolomite rhombs from the Fort Payne Formation (Lower Mississippian) of western Tennessee. E, Chert clast in which a small amount of iron oxide stain highlights the abundant dolomolds. F, Sand-sized grains of chert from the Tuscaloosa Gravel.

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and cannot be equivalent. On the other hand, the Mounds Gravel of southern Illinois has a composition similar to that of the UC and, furthermore, is similar in hand specimens; it is the UC by another name. One of the more remarkable aspects of the UC is the overall similarity of its sedimentary structures throughout the Mississippi Embayment (fig. 3; table 2; Dockery et al. 2008) and the similarity in the relative abundance of bedded and nodular chert. However, there are differences in the sand fraction; these suggest differences in sources for the Chickasaw Bluffs versus Crowley’s Ridge. Potter (1955a) noted a difference in heavy mineral content, which was confirmed here (table 4). Furthermore, both sand composition and grain size suggest differences in sources for the two areas. Chickasaw Bluffs sand is dominated by monocrystalline quartz with some polycrystalline quartz, lesser chert, and no feldspar (fig. 8A). This contrasts with Crowley’s Ridge samples, which contain less monocrystalline quartz, a greater proportion of polycrystalline quartz, and, most importantly, 5%–7% feldspar (both plagioclase and K-spar; fig. 8B). The median grain size of the sand fraction also differs; Chickasaw Bluffs sand is coarser than Crowley’s Ridge sand (fig. 8C). The hypothesis of a Pliocene drainage basin extending into southern Canada was tested using detrital zircon ages from the UC. The sand fraction of Phanerozoic clastic rocks that may have covered Pliocene southern Canada would have contained zircons reworked from underlying Archean rocks. If the PMR basin extended into Canada and transported sediment derived from this preshield (preglaciation) terrain covered with Phanerozoic strata, the sand fraction of the UC should contain reworked Archeanage zircons (12.5 Ga). Figure 9 shows zircon ages with a strong mode at ∼1000 Ma; secondary modes at ∼400, 1200, 1400, and 1700 Ma; and some Archean dates (12.5 Ga). Grenville Province rocks presently exposed in the southeastern Canadian Shield and the Adirondacks, as well as basalts of the Central North America Rift System, are about 1000 m.yr. old (Greer 1995; Tollo et al. 2004), suggesting that they are the source for zircons of the strong mode. The sources of the secondary modes may be the Appalachians, Granite/Rhyolite Province, and Central Plains (Yavapai/Mazatzal) orogen. More importantly, from our perspective, Archean-age zircons (12500 Ma) point to a source in the central Canadian Shield. Discussion Our first focus is on the source of the gravel, and then we discuss the source of the sand as a sepa-

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Figure 5. Thin-section photomicrographs of nodular chert clasts taken with uncrossed polarizers. In A, the central clast is composed of chert-replaced intraclasts. In B, the central clast is composed of chert-replaced oolites. In C, the clast is a chert-replaced stromatoporoid. Note also that the black goethite cement (G) coated the grain before the kaolinite cement (K) was precipitated.

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Figure 6. Thin-section photomicrographs of sand grains. A–C were taken with uncrossed polarizers, and D was taken with crossed polarizers. A, Sharply angular embayed grain of limpid quartz (E p epoxy with dispersed fine-grained silicon carbide; F p feldspar; H p hole in epoxy; Q p quartz grain). B, Detrital quartz grain coated with a dust rim that was subsequently coated with quartz in optical continuity with the detrital grain. C, Sand-sized

rate, albeit related, issue. Middle Paleozoic Silurian, Devonian, and Mississippian fossils are abundant in chert clasts of the UC. Extensive exposures in Kentucky, Indiana, and Ohio were a probable source for UC gravel in the study area; however, the similarity of the Mounds Gravel and the UC suggests that some UC gravel came from north of the present junction of the Ohio and Mississippi Rivers, latitude ∼377 (Nelson and Williams 2004; Nelson 2010; Cupples and Van Arsdale 2014). Contemporary midcontinent exposures of middle Paleozoic bedded chert and carbonates are extensive in the Upper Mississippi River valley in Illinois, Missouri, and Iowa (Cook and Bally 1975; fig. 1). Although some UC gravel in the study area undoubtedly came from relatively proximal sources, the huge volume of UC gravel, much of it nodular chert from dominantly carbonate formations, requires an enormous volume of sedimentary source rocks. The geologic map of North America (Reed et al. 2005) shows Ordovician, Silurian, and Devonian carbonates along the southwestern margin of Hudson Bay that are currently separated from lithologically similar Ordovician, Silurian, and Devonian units (albeit with more clastics) near Lake Winnipeg by 700 km of Archean bedrock of the Severn Arch (fig. 1). The similarity of these Paleozoic lithologies strongly suggests that the systems were once continuous (Andrichuk 1959; Ambrose 1964; Meneley 1964; Norris and Sanford 1968; Sanford et al. 1968; Liberty and Bolton 1971; Crowley and Kuhlman 1988; Pinet et al. 2013). On the basis of the presence of marine Cretaceous sediments in Hudson Bay and Ontario, Williams and Stelck (1975) and White et al. (2000) proposed that the Cretaceous Interior Seaway extended across Manitoba and Ontario, and we concur. Marine Cretaceous sediments are preserved only in contact with Devonian strata in Hudson Bay and the Moose River basin of eastern Ontario southwest of James Bay. Marine Cretaceous sediments are absent, even as outliers, on all pre-Devonian rocks (Silurian, Ordovician, and Archean) to the west of Lake Winnipeg in southwestern Manitoba, where, again, marine Cretaceous sediments overlie Devonian rocks (Reed et al. 2005). The presence of marine Cretaceous sediments on Devonian rocks and their absence on pre-Devonian strata suggests that the marine Cretaceous sediments were deposited before pre-Devonian rocks were exposed. Moreover, it suggests that Devonian as well as Silurian and Ordo-

clast composed of fine siltstone. D, Polycrystalline quartz grain (left) and grain of plagioclase feldspar (right).

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Figure 7. A, Close-up photo of the common kaolinite clay clasts. B, Thin-section photomicrograph of a squeezed kaolinite clay clast. The clast fills intergrain space. Note that there is no secondary kaolinite on the quartz grains in contact with the clast, whereas secondary kaolinite (dark) coats grains that abut pores. BDE p blue-dyed epoxy; P p pore; Q p Quartz grain.

vician strata were continuous across Manitoba and Ontario prior to deposition of Cretaceous sediments, in agreement with common textbook paleogeographic reconstructions that depict Manitoba and Ontario as an area of shallow marine carbonate deposition during the middle Paleozoic. Along the margins of the Severn Arch, Jurassic impact crater fill (Short 1970) and Cretaceous alluvium (Zippi and Bajc 1990) in contact with Archean basement, along with Neogene lacustrine strata (which we interpret as a sinkhole deposit) on Ordovician carbonate bedrock (Galloway et al. 2012), give evidence of local erosion through the Paleozoic

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cover prior to the Pleistocene. On the basis of these three sites, we suggest that there was considerable relief (hundreds of meters) on the Paleozoic and Mesozoic cover of the Archean basement of the Severn Arch prior to Pleistocene glaciation. Cox et al. (2014) summarized prior research and pointed out that the oldest North American Pleistocene tills have a significantly lower percentage of crystalline rock clasts than younger tills. This is consistent with early Pleistocene glacial advances across Phanerozoic strata that ultimately stripped the Phanerozoic sedimentary cover from the shield, exposing Archean rocks as a major source of clasts in younger tills (Meneley 1964). The abundant gravel clasts of the Pliocene UC, derived from Paleozoic sedimentary rocks, contrast with the Archean igneous/metamorphic gravel clasts common in Pleistocene tills, although geographically the source areas may have been the same. What feature might there be within the Paleozoic clasts of the UC that could signal their source? Plate tectonic reconstructions show North America at the equator and rotated clockwise 507–907 from its current north-south orientation during the Paleozoic. Thus, there would be no paleotemperature-related gradient in early to middle Paleozoic fossils from Hudson Bay to the Gulf of Mexico. Given the absence of such a gradient, the relative contribution of Canadian versus nearby sources to UC gravel is difficult to estimate. The source of the sand is a problem related but not identical to the source of the chert gravel. Neither bedded neither chert nor carbonate rocks (the source of nodular chert) yield large quantities of monocrystalline quartz sand; thus, we must look elsewhere for the major sources of sand. Some may have been eroded from the underlying Paleogene Cockfield For- mation or Memphis Sand (the poorly exposed contact is erosional); however, this is not a major source because their very fine pure-quartz sand contrasts with the coarse, commonly cherty, and more heterogeneous sand of the UC. More likely sources of UC monocrystalline quartz sand are regionally extensive Paleozoic sandstones, such as the Cambrian Mount Simon and Ordovician Saint Peter. The presence of two feldspars and volcanic quartz in UC sand from Crowley’s Ridge strongly suggests a contribution from the immediate northwest, the granitic and rhyolitic Saint Francois Mountains of southeastern Missouri (as does the presence of Potosi Agate in UC gravel). Abundant polycrystalline quartz with complex suture grain boundaries suggests a metamorphic source, such as the Superior and Trans-Hudson provinces of the northern United States and southern Canada.


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Table 3.

Grain Count Data for Upland Complex Sand (%)

Location Crowley’s Ridge, Arkansas: Levesque Levesque Levesque Levesque Chickasaw Bluffs, Tennessee: Shelby Shelby Chickasaw Bluffs, Mississippi: DeSoto DeSoto DeSoto DeSoto Panola Panola Tuscaloosa Gravel, Tennessee: Tuscaloosa, TN Tuscaloosa, TN Tuscaloosa, TN Tuscaloosa, TN Tuscaloosa Gravel, Kentucky: Tuscaloosa, KY Tuscaloosa, KY Tuscaloosa, KY Tuscaloosa, KY Mounds Gravel, Illinois: Mounds Mounds Mounds Mounds Grover Gravel, Missouri: Grover Grover Grover Grover

Num

mm

17 17 17 17

1.71 .71–.5 .5–.25 !.25

9 9

Mid Phi

Mono

Poly

Cht

Op

Feld

!.5 .75 1.5 2.5

70 58 44 41

12 26 34 24

12 9 6 17

0 1 12 11

6 6 4 7

.71–.5 .5–.25

.75 1.5

87 41

9 21

2 22

2 16

0 0

5 5 5 5 2 2

1.71 .71–.5 .5–.25 !.25 .71–.5 .5–.25

!.5 .75 1.5 2.5 .75 1.5

92 86 58 26 92 63

4 5 19 9 4 22

4 6 11 32 4 13

0 3 12 33 0 2

0 0 0 0 0 0

19 19 19 19

2.0–4.0 1.0–2.0 .5–1.0 .25–.5

21.5 2.5 .5 1.5

0 0 1 15

2 0 0 0

98 100 99 85

0 0 0 0

0 0 0 0

20 20 20 20

2.0–4.0 1.0–2.0 .5–1.0 .25–.5

21.5 2.5 .5 1.5

1 2 6 11

1 0 0 0

86 74 84 79

12 24 10 10

0 0 0 0

21 21 21 21

2.0–4.0 1.0–2.0 .5–1.0 .25–.5

21.5 2.5 .5 1.5

0 ... 72 83

18 . .. 15 8

39 ... 4 5

43 ... 9 4

0 ... 0 0

22 22 22 22

2.0–4.0 1.0–2.0 .5–1.0 .25–.5

21.5 2.5 .5 1.5

26 51 67 73

46 22 8 11

13 13 9 8

0 0 0 0

15 14 19 8

Note. Num p location number in table 2; Mono p monocrystalline quartz; Poly p polycrystalline quartz; Cht p chert; Op p opaque, partial to complete replacement of chert by goethite; Feld pfeldspar, combined plagioclase and orthoclase.

The pre-Pleistocene landscape of northern North America was strongly modified by glacial processes, making assessment of the Pliocene drainage patterns in Canada uncertain. However, a substantial literature posits the presence of a Paleogene and early Neogene river system (Bell River) with its headwa-

Table 4.

ters in the western Canada Cordillera that drained east across what is now Hudson Bay to the Labrador Sea (Duk-Rodkin and Hughes 1994; Cummings et al. 2012; Sears 2013; Bentley et al. 2016). We propose that the headwaters of the PMR extended much farther north than the source of the modern Mis-

Percent Heavy Minerals

Location Present study: Shelby Co., Tennessee Panola Co., Mississippi Grover Gravel Potter 1955a, table 7: Chickasaw Bluffs, W Kentucky Crowley’s Ridge Mounds, Illinois

Num

Kyanite Rutile

Staur

Titan

Silliman

Topaz

Tour

Zircon

Other

9 6 23

23 25 5

25 15 8

2 1 3

5 5 5

2 0 7

0 0 0

1 1 0

22 33 56

20 20 16

12 13 21

48 8 14

4 4 4

19 25 9

0 0 0

18 5 5

0 2 0

2 16 16

9 52 52

ND ND ND

Note. Locations for Potter samples are estimates. Num p location number in table 2; Staur p Staurolite; Titan p Titanite; Silliman pSillimanite; Tour pTourmaline; ND pnot determined.

Journal of Geology


sissippi River and captured some of the northeastflowing Bell River drainage (fig. 1). The HatfieldSpiritwood system is a southeast-flowing preglacial river system that crosscuts older channels (Bell River? Pugin et al. 2014). A northeast to southeast downstream change in the course of the Hatfield-Spiritwood system suggests that its northeast-trending reaches were tributaries of the Bell River system that were captured by southeast-flowing Hatfield-Spiritwood rivers that flowed southerly into the PMR (fig. 1). A topographic profile following the path of the modern Mississippi River and projected into southwestern Ontario illustrates a divide at ∼400 m separating the present Mississippi River headwaters from Hudson Bay drainage (Cupples and Van Arsdale 2014, their fig. 9B). The area of the Severn Arch between Hudson Bay and Lake Winnipeg (fig. 1) is ∼200 m in elevation at present. For the PMR to have drained southwestern Ontario and adjacent Manitoba, a divide on the Severn Arch between Hudson Bay and Lake Winnipeg must have been higher in elevation than the location of the present divide in southwestern Ontario during the Pliocene. Is this possible? It is reasonable to assume that the Pleistocene continental ice sheets would have originated at a topographically high elevation, thus suggesting that the Keewatin and Labrador ice accumulation centers flanking modern Hudson Bay were topographic highs during the Pliocene. This assumption is supported by the observation that the Hudson Bay region is currently undergoing isostatic uplift with rates reaching ∼10 mm/yr (Sella et al. 2007) and is estimated to have several hundred meters farther to rise until it reaches isostatic equilibrium in ∼30–40 k.yr. (Watts 2001). This leads to the conclusion that the Hudson Bay depression did not exist during the Pliocene, and the region was above sea level. The area of the Severn Arch—the area of most interest here— is also undergoing uplift (Sella et al. 2006) and must have been at a higher elevation than at present during the Pliocene prior to Pleistocene glacial loading. As argued above, Ordovician, Silurian, and Devonian strata (largely carbonate) exposed on the western margin of Hudson Bay were probably continuous with similar age and lithology strata under and west of Lake Winnipeg in southeastern Manitoba. The total thickness of the Paleozoic section currently on the Hudson Bay margin is ∼760 m (Norris and Sanford 1968) and 600 m near Lake Winnipeg (Andrichuk 1959). Thus, some 700 m of Paleozoic strata may have overlain Archean bedrock on the Severn Arch. This thickness estimate is conservative; apatite fission-track thermal history models suggest that 1000 m of Paleozoic strata were initially present

513

Figure 8. Plots comparing the composition of sandsized grains on the Chickasaw Bluffs versus Crowley’s Ridge. A, Chickasaw Bluffs (location 4 of fig. 2 and table 2). B, Crowley’s Ridge (location 14 of fig. 2 and table 2). C, Cumulative curve plots of grain size data. The circle highlights the median grain size for samples from the Chickasaw Bluffs, and the rectangle highlights that for Crowley’s Ridge samples. Crs p coarse; Med p medium coarse; VC p very coarse; V fine p very fine.

on the southwestern edge of the Severn Arch (Feinstein et al. 2009), and early Paleozoic carbonates may be 2500 m thick in the middle of Hudson Bay (Norris


515 14

Figure 9. Zircon age data. The various modes are labeled as to possible sources. CNARS p Central North America Rift System (aka midcontinent rift).

and Sanford 1968; Pinet et al. 2013; Lavoie et al. 2015). At the margins of the Severn Arch, the Paleozoic/Mesozoic unconformity preserves paleovalleys and karst depressions showing hundreds of meters of relief (Norris et al. 1982; MacNeill et al. 2011). Furthermore, marine Mesozoic and Paleogene cover across the Severn Arch may have been up to 1400 m thick prior to Pleistocene glacial erosion (Feinstein et al. 2009). Thus, the Pliocene landscape of the Severn Arch may have been that of a high-relief dissected plateau underlain by several hundred meters or more of Ordovician to Cenozoic marine strata. Some canyons and karst sinkholes may have cut through much or all of the sedimentary sequence. For example, Cretaceous alluvium on Archean basement reported by Zippi and Bajc (1990) may have been deposited in the bottom of a relatively deep canyon system. We propose that the northern divide of the PMR basin was within this plateau topography in the central Severn Arch. Summary We feel that our studies in this and earlier publications provide a factual basis in support of Gilbert’s hypothesis—that the Saskatchewan River was formerly the headwaters of the Mississippi River (Gilbert 1877, p. 126). On the basis of grain size, composition, and architectural elements, we believe that the UC (middle Pliocene), in the northern Missis-

sippi Embayment region, is a gravel and sand fluvial terrace deposit of the PMR system. The present extent of the UC is the remnant of a deposit that may have stretched from a source region near what is now Hudson Bay, Canada, south to the Gulf of Mexico (2500 km). Although there is no unmistakable evidence in the petrology of the UC that proves a Canadian source, our data are consistent with prior studies that concluded that the headwaters of the PMR extended into southern Canada. The coarse gravel and overall similarity in architectural style throughout the present length of UC exposures (a straight-line distance of 700 km from southern Illinois to southern Mississippi) implies an enormous discharge. The absence of carbonate clasts of any size in the UC, despite the fact that a major proportion of the chert clasts came from sedimentary carbonates, suggests a warm climate. This is consistent with the abundant contemporaneous petrified wood and the abundance of goethite/limonitestained secondary kaolinite cement in the UC, assumed to be a product of soil-forming processes in a humid climate. The UC is the only stratigraphic unit in midcontinental North America that was deposited between the end of the Eocene epoch (∼33.9 Ma) and the start of the Pleistocene epoch (∼2.58 Ma; table 1). Clearly, the UC can provide only a glimpse of the conditions that existed in the nearly 30-m.yr. hiatus; however, it is the only glimpse available, and therefore the UC provides essential ground truth about environmental conditions toward which our present environment may be trending. Conclusions 1. An analysis of the UC gravel and sand did not uniquely identify its provenance. 2. The composition of the UC is not similar to that of the chert gravel of the Late Cretaceous Tuscaloosa Formation, although Tuscaloosa exposures are close to the study area. 3. The composition of the UC is not similar to that of the Grover Gravel or other Pleistocene gravels. 4. The composition of the UC is very similar to that of the Mounds Gravel, a deposit found along the Mississippi River valley, perhaps as far north as northernmost Illinois. 5. We speculate that much of the chert gravel in the UC came from early to mid-Paleozoic formations as far away as southcentral Canada. The relative proportion of Canadian versus proximal US sources is unknown. 6. Some sand may have come from the same formations as the chert and from Paleozoic sandstones

Journal of Geology


associated with the carbonates that covered southwestern Ontario and adjacent Manitoba during the Pliocene. However, the texture and composition of UC sand points to proximal and multiple overlapping sources, including the Saint Francois Mountains of Missouri, as well as more distant sources, including the Grenville Terrane of the Appalachian Blue Ridge. 7. The present extent of the UC is the vestigial remnant of an umbilical cord of gravel and sand that once extended from Canada to the Gulf Coast following the general path of the modern Mississippi River. Sediment transported down subtill valleys in western Canada appear to have connected to the headwaters of the PMR. Similar valleys may well have existed in central Canada prior to glaciation. 8. We conclude that the PMR probably drained a much larger area than the modern Mississippi River,

515

an area that extended well north of the US-Canada border into Manitoba and southwestern Ontario. A C K N O W L E D G M E N TS

We are grateful to geologists A. Parks, Memphis Stone and Gravel; W. Goodwin, APAC Corporation; L. Kirk, Old Hickory Clay Company; and C. Hunt, Standard Construction Company, for guiding us into company quarries. G. Swihart identified goethite in thin sections using infrared reflectance microscopy, and D. Larsen provided X-ray diffraction analyses of fines. We thank C. Rovey and M. Roy for sending us unpublished details of their grain composition data and Robert Hatcher for age dates of zircons. This article benefited greatly from the comments of reviewers M. Guccione and E. Otvos as well as an anonymous reviewer.

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