geology and geomorphology of the carolina sandhills, chesterﬁ … · 2016-12-21 · field guide...

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The Geological Society of AmericaField Guide 42

Geology and geomorphology of the Carolina Sandhills, Chesterfi eld County, South Carolina

Christopher S. SwezeyU.S. Geological Survey, 12201 Sunrise Valley Drive, MS 926A, Reston, Virginia 20192, USA

Bradley A. FitzwaterG. Richard Whittecar

Old Dominion University, Department of Ocean, Earth, and Atmospheric Sciences, 4600 Elkhorn Avenue, Norfolk, Virginia 23529, USA

ABSTRACT

This two-day fi eld trip focuses on the geology and geomorphology of the Caro-lina Sandhills in Chesterfi eld County, South Carolina. This area is located in the updip portion of the U.S. Atlantic Coastal Plain province, supports an ecosystem of longleaf pine (Pinus palustris) and wiregrass (Aristida stricta), and contains three major geologic map units: (1) An ~60–120-m-thick unit of weakly consolidated sand, sandstone, mud, and gravel is mapped as the Upper Cretaceous Middendorf Formation and is interpreted as a fl uvial deposit. This unit is capped by an uncon-formity, and displays reticulate mottling, plinthite, and other paleosol features at the unconformity. The Middendorf Formation is the largest aquifer in South Caro-lina. (2) A 0.3–10-m-thick unit of unconsolidated sand is mapped as the Quater-nary Pinehurst Formation and is interpreted as deposits of eolian sand sheets and dunes derived via remobilization of sand from the underlying Cretaceous strata. This unit displays argillic horizons and abundant evidence of bioturbation by veg-etation. (3) A <3-m-thick unit of sand, pebbly sand, sandy mud, and mud is mapped as Quaternary terrace deposits adjacent to modern drainages. In addition to the geologic units listed above, a prominent geomorphologic feature in the study area is a north-trending escarpment (incised by headwater streams) that forms a mark-edly asymmetric drainage divide. This drainage divide, as well as the Quaternary terraces deposits, are interpreted as evidence of landscape disequilibrium (possibly geomorphic responses to Quaternary climate changes).

Swezey, C.S., Fitzwater, B.A., and Whittecar, G.R., 2016, Geology and geomorphology of the Carolina Sandhills, Chesterfi eld County, South Carolina, in Doar, W.R., III, ed., [[Title Title Title Title Title Title Title Title Title Title Title Title Title Title Title]]: Geological Society of America Field Guide 42, p. 1–28, doi:10.1130/2016.0042(02). © 2016 The Geological Society of America. All rights reserved. For permission to copy, contact [email protected].

2 Swezey et al.


INTRODUCTION

This fi eld trip provides an overview of the geology and geo-morphology of the Carolina Sandhills in Chesterfi eld County, South Carolina. Most of the data presented in this paper are based on recent detailed geologic maps of the Middendorf and Patrick quadrangles of Chesterfi eld County (to be published by the South Carolina Geological Survey). These two quadrangles encompass most of the Carolina Sandhills National Wildlife Refuge and the Sand Hills State Forest, and contain the following three major geologic map units:

(1) An ~60–120 m- (197–394-ft-) thick unit of weakly consoli-dated sand, sandstone, mud, and gravel that is mapped as the Upper Cretaceous Middendorf Formation. The unit has several distinct lithologic facies, and it extends to the east in the subsurface, where it forms one of the larger aquifers in the southeastern United States. This unit is interpreted as a fl uvial deposit that accumulated during a base-level fall (lowstand) and subsequent early transgression.

(2) A 0.3–10-m- (1–33-ft-) thick unit of unconsolidated sand that is mapped as the Quaternary Pinehurst Formation. This unit forms the “sandhills” of the region, and it extends across most of the Middendorf and Patrick quadrangles. This Quaternary sand is interpreted as eolian sand sheets and dunes derived from the underlying Cretaceous Mid-dendorf Formation during the last glaciation when climate conditions were colder, windier, and more arid.

(3) A <3-m- (10-ft-) thick unit of sand, pebbly sand, sandy mud, and/or mud that is mapped as Quaternary terrace deposits adjacent to the modern drainages.

The three map units listed above have different geomorpho-logical expressions and have been subjected to different pedogenic processes, many of which are discussed during this fi eld trip.

The Carolina Sandhills is a 15–60-km- (9–37-mi-) wide physiographic region of rolling sandy topography that extends from the western border of Georgia to central North Carolina along the updip (north and west) margin of the Coastal Plain province in the southeastern United States (Fig. 1). The rec-ognition of the Carolina Sandhills as a separate physiographic region dates from at least the work of McGee (1890, 1891), who described sand hills on the inner coastal plain of South Caro-lina and North Carolina. Holmes (1893) delineated the “sand-hill country of the Carolinas” as a distinct geological region of the inner coastal plain between the Savannah River of South Caro-lina and the Neuse River of North Carolina, and Darton (1896) noted that the sandhill sediments extend across both South Caro-lina and Georgia. Cooke (1936) later used the term “Congaree Sand Hills” to denote the extensive sand accumulations across the inner coastal plain of South Carolina.

Relatively few descriptions have been published on the Caro-lina Sandhills in South Carolina, and age estimates have been rather tentative. In fi eld trip guides for Richland and Kershaw Counties, Johnson (1961) and Otwell et al. (1966) provided brief descriptions of the sandhills, which they referred to as being of “post-Eocene”

age. In a geologic map of the Blaney quadrangle in Richland and Kershaw Counties, Ridgeway et al. (1966) described the sandhills as uniform sand of probably Tertiary age. In Lexington and Aiken Counties, Kite (1987) provided brief descriptions of the sandhills, and stated that their age was unknown. In a summary paper cover-ing both South Carolina and North Carolina, Nystrom et al. (1991) provided succinct descriptions of the sand thickness, topographic expressions, and elevations, and estimated the sand to be of late Miocene age. In Chesterfi eld County, Leigh (1998) described six soil profi les among the sandhills, and estimated the surfi cial sand to be of Tertiary age.

Along with uncertain age estimates, the depositional envi-ronment of the Carolina Sandhills has been the subject of much speculation. In South Carolina, speculation on the depositional environment of the surfi cial sand has ranged from eolian to sub-aqueous, and some researchers have postulated specifi c subaqueous environments ranging from fl uvial to marine (Cooke, 1936; Ridge-way et al., 1966; Kite, 1987; Nystrom and Kite, 1988; Nystrom et al., 1991; Leigh, 1998). Many of the statements about depositional environments, however, have been simply conjectures without much descriptive information to support the interpretations.

Despite these uncertainties, several authors have noted a close association of the Carolina Sandhills with outcrops of underlying Cretaceous sand and sandstone. Cooke (1936), for example, noted that the area of the Congaree Sand Hills corre-sponds to the area in which the Cretaceous Tuscaloosa Forma-tion (what is now called the Middendorf Formation) is exposed. Ridgeway et al. (1966) noted that the sandhills in Richland and Kershaw Counties overlie sand of the Cretaceous Middendorf Formation, and they stated that the two units have similar grain sizes, as well as similar abundance and composition of heavy minerals. They speculated that the overlying sand (“sandhills”) was derived from reworking of the underlying Cretaceous sand.

ROAD LOG AND STOP DESCRIPTIONS

This fi eld trip is focused on the Carolina Sandhills region in Chesterfi eld County, South Carolina (Fig. 2). In this county, the sandhills occur on a relatively high plateau of Cretaceous strata, bounded to the west by Paleozoic schist of the Piedmont province and bounded to the east by the east-facing Orangeburg Scarp (Swift and Heron, 1969). This scarp was named for a site at the City of Orangeburg in Orangeburg County (South Carolina), where the toe of the scarp lies at ~64 m (210 ft) above sea level (Colquhoun, 1962). In Figure 2, the Orangeburg Scarp is a north-northeast–trending linear feature visible in western Lee County and western Darlington County, where the topography changes abruptly from ~60–80 m (197–262 ft) elevation (green, yellow) to 100–120 m (328–394 ft) elevation (orange, red). The Orangeburg Scarp is interpreted as the shoreline formed by wave erosion at a time of high sea level during the middle Pliocene (Dowsett and Cronin, 1990). East of this shoreline lie extensive low-relief Coastal Plain terraces (composed of coastal and marine sediments) that are domi-nated by oval topographic depressions called Carolina Bays, and

Geology and geomorphology of the Carolina Sandhills, Chesterfi eld County, South Carolina 3


related geomorphological features formed by eolian processes (e.g., Ivester et al., 2002, 2003; Moore et al., 2014). West of the Orange-burg Scarp, no broad low-relief surfaces exist where Carolina Bays could form, and eolian sand sheets and dunes formed on the rolling, dissected uplands of the “sandhills” region.

There are ten stops scheduled for this fi eld trip (Fig. 3). The fi rst six stops are scheduled for Day 1, and are located in the western portion of the study area. The four stops scheduled for Day 2 are located in the eastern portion of the study area.

DAY 1 (2 APRIL 2016)Drive from Columbia to the Carolina Sandhills National Wildlife Refuge.

Upon arrival at the Carolina Sandhills National Wildlife Ref-uge, the day begins with an overview of the NWR, followed by a stop at an outcrop of the Cretaceous Middendorf Formation, and a stop among the Quaternary eolian dunes (“sandhills”) that are now stabilized by vegetation. After lunch at Lake Bee and a

COASTAL PLAIN

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Figure 1. Location of the Carolina Sandhills in the southeastern United States. Outline of the Carolina Sandhills is from Griffi th et al. (2001, 2002). Locations of the Cape Fear Arch, Peninsular Arch, and Ocala Arch are from LeGrand (1961). Outline of Eocene crater is from Powars et al. (2015). The Middendorf quadrangle is immediately to the west of the Patrick quadrangle.

4 Swezey et al.


look at nearby outcrops of the Cretaceous Middendorf Forma-tion, there is a stop at an outcrop displaying extreme pedogenic alteration (“plinthite”) at the top of the Middendorf Formation, and another stop near the Middendorf type section at a pit where the U.S. Geological Survey (USGS) obtained a 85.3 m (280 ft) deep core in the Middendorf Formation.

STOP 1: Headquarters of Carolina Sandhills National Wildlife Refuge in Middendorf Quadrangle (N 34° 30′ 13.48″, W 80° 13′ 29.92″)

Many of the plant species of the Carolina Sandhills show distinct associations with the underlying geology. For example,

pine trees in the southeastern United States generally prefer sandy substrates, and the names of many of the towns in the Carolina Sandhills region have some form of the word “pine” (e.g., Pinehurst, Southern Pines). Within the Carolina Sandhills of Chesterfi eld County, the predominant substrates are sand of the Quaternary Pinehurst Formation and the underlying Cre-taceous Middendorf Formation. Accordingly, the vegetation of the refuge is dominated by trees of longleaf pine. There is also a scattered understory of scrub oak (turkey oak, dwarf post oak), and a groundcover of wiregrass (Christensen, 2000; Ear-ley, 2004; Sorrie, 2011). On outcrops of clay (within the Cre-taceous Middendorf Formation), however, blackjack oak may be more common than other trees (Askins, 2010). In addition,

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Figure 2. Shaded relief map of Kershaw, Chesterfi eld, Lee, and Darlington Counties, South Carolina (Albers projection, 1983 North American Datum). Two-meter elevation data are derived from LiDAR point cloud data (available on-line at www.dnr.sc.gov/GIS/lidar.html, accessed 19 July 2015). The Orangeburg Scarp is a north-northeast–trending linear feature visible in western Lee County and western Darlington County, where the topography changes abruptly from ~60–80 m (197–262 ft) elevation (green, yellow) to 100–120 m (328–394 ft) elevation (orange, red). Black rectangles show the locations of the Middendorf quadrangle on the west and the Patrick quadrangle on the east, and both quadrangles are shown in greater detail in Figure 3. Blue circles show locations of cores discussed at Stop 6. NC—North Carolina; SC—South Carolina.



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6 Swezey et al.


pond cypress and sweetgum are present on terraces adjacent to some of the creeks.

At this stop, we discuss the ecological signifi cance and his-tory of the Carolina Sandhills National Wildlife Refuge, a region known for an ecosystem of longleaf pine (Pinus palustris) and wiregrass (Aristida stricta). Longleaf pine is able to grow in a wide variety of settings, and the “sandhills” is just one type of longleaf pine community (Fig. 4). The longleaf pine and wire-grass ecosystem is maintained by frequent low-intensity fi res that facilitate the reproduction of the trees, wiregrass, and asso-ciated plants (Earley, 2004; Askins, 2010). The longleaf pine is particularly well adapted to fi re because of features such as thick bark, large seed size, inconsistent seeding, fall seed sprouting, and slow growth during the early years of the tree. Many details of the role of fi re in this ecosystem are not well understood, but it is known that fi re reduces the amount of ground litter, reduces competition from other tree species, returns nutrients to the soil, and inhibits certain pathogens that affl ict longleaf pine and other associated species (Earley, 2004; Askins, 2010).

As described in Earley (2004) and Finch et al. (2012), the longleaf pine forests and savannas of the southeastern United States once formed one of the more extensive woodland eco-systems in North America. When the Spanish arrived during the early 1500s, longleaf pine was the dominant tree that cov-ered ~60% of the Atlantic and Gulf Coastal Plains, covering a wide swath of every coastal state from the James River in Virginia to the shores of Lake Okeechobee in Florida and west into southeastern Texas. The longleaf pine was the dominant tree over ~242,800 km2 (93,745 mi2) of the southeast, and the longleaf pine occurred together with other pines and hard-woods on an additional 121,400 km2 (46,873 mi2). Today, how-ever, only 1.4% of the Atlantic and Gulf Coastal Plains support longleaf pine. In 1996, it was estimated that only 11,938 km2 (4609 mi2) remained of longleaf pine forest, and much of this remaining area consisted of small fragments of land (Earley, 2004). In other words, the longleaf pine ecosystem has experi-enced a decline of ~95%.

The longleaf pine is exceptionally strong and resistant to rot, and the tree was harvested for timber (especially ship build-ing). The longleaf pine was also a major source of tar, pitch, and turpentine. Prior to 1860, however, the cutting of longleaf pine timber and getting it to market was a slow and diffi cult process. By the time of the American Civil War (1861–1865), the longleaf pine forest was essentially extirpated from Virginia and north-eastern North Carolina, but much of the longleaf pine forest to the south remained relatively intact. After 1865, however, the lumber industry focused on the southeastern United States and with the use of railroads, essentially destroyed the remaining longleaf pine forest within 50 years (Earley, 2004).

During the 1930s, the U.S. Federal Government purchased agriculturally poor and/or damaged land in the Sandhills region of Chesterfi eld County (South Carolina), and in 1939, established this land as the Carolina Sandhills National Wildlife Refuge to be man-aged by what is now the U.S. Department of the Interior Fish and

1Unless otherwise stated, the color nomenclature used in this guide is from the Geological Society of America rock color chart by Goddard et al. (1963).

Wildlife Service (Askins, 2010). The original purposes of the ref-uge were to provide habitat for migratory birds, to provide wildlife-dependent recreation opportunities, and to demonstrate land man-agement practices that would enhance the conservation of natural resources. During the 1950s and 1960s, loblolly pine and slash pine (which are not native) were planted to replace much of the longleaf pine. Today, however, the loblolly pine and slash pine are being har-vested, these areas are being replanted with longleaf pine, and the refuge is managed primarily to restore and maintain the longleaf pine and wiregrass ecosystem (Askins, 2010).

STOP 2: Outcrop of Middendorf Formation in Pit on Old Wire Road within the Carolina Sandhills National Wildlife Refuge in Middendorf Quadrangle (N 34° 31′ 36.89″, W 80° 12′ 57.71″)

Stop 2 is a borrow pit located off Old Wire Road (Fig. 5) that displays good exposures of the Middendorf Formation. This road is the remnants of an old stage-coach route along which was strung an early telegraph wire line. During the Civil War, General William T. Sherman’s Union army marched along Old Wire Road from Columbia to Cheraw and eventually into North Carolina.

The pit at Stop 2 is one of the better outcrops in the wild-life refuge, and it shows the predominant Middendorf lithology of grayish-red (5R 4/2) to dark yellowish-orange (10YR 6/6) medium to coarse sand or sandstone.1 At most locations, the sand or sandstone is moderately to poorly sorted. The degree of lithifi -cation is quite variable over short distances, ranging from loose sand, to sand or sandstone held together by kaolin matrix, to iron-cemented sandstone.

Fossils are not common in the Middendorf Formation. Berry (1910, 1914) identifi ed fossil leaves in a clay bed at the Midden-dorf type section (near Stop 6), and estimated the age of the leaves to be early Late Cretaceous (possibly Cenomanian–Turonian). Pieces of petrifi ed wood have been identifi ed at several outcrops in the Middendorf and Patrick quadrangles, and some of these pieces have been identifi ed as conifer wood (Fitzwater et al., 2014a). A core in the Middendorf Formation at Cheraw State Park, which is ~32 km (20 mi) northeast of Stop 2, has yielded Late Cretaceous palynomorphs including abundant fern spores (Cicatricosisporites, Gleicheniidites, Foveotriletes), as well as gymnosperm pollen (Taxodiaceae and Pinuspollenites), angio-sperm pollen (Momipites), and freshwater algae (Schizosporis). These palynomorphs were identifi ed by Christopher Bernhardt (USGS), and are reported in Swezey et al. (2015).

On the east wall of the pit at Stop 2 (Fig. 6), the following two stratigraphic units are apparent: (1) a lower unit of medium to coarse sand with east-dipping cross-bedding; and (2) an upper unit of medium to coarse sand with approximately horizontal planar bedding. The sand-sized grains consist predominantly of



quartz with some mica and opaque minerals, and in places, there is a matrix of kaolin clay among the sand grains. In addition, the lower unit with cross-bedding contains some clasts of kaolin clay.

On the south wall of the pit (Fig. 7), the same two strati-graphic units are apparent, and beds (lenses) of clay are also vis-ible. An ~3-m- (9.8-ft-) long by 0.3-m- (1.0-ft-) thick clay bed (lens) is located at the contact between the lower unit with cross-bedding and the upper unit with horizontal planar bedding. Clay beds (lenses) of this size are relatively common within the upper portion of the Middendorf Formation, but core data (seen at Stop

6) reveal that such clay beds become increasingly rare lower in the formation.

On the west wall of the pit (Fig. 8), the strata consist of medium to coarse sand composed predominantly of quartz, but primary sedimentary structures are not apparent and separate depositional units cannot be distinguished. This outcrop in which sedimentary structures are not apparent is typical of most out-crops of the Middendorf Formation.

The strata exposed at Stop 2 are interpreted as fl uvial sediments, primarily channel deposits. The scarcity of distinct fi ning-upward

Figure 4. Longleaf pine (Pinus palus-tris) in sandy soil of the Carolina Sand-hills National Wildlife Refuge. Circl ed twenty-eight-cm- (11-in.-) long hammer at base of small tree in foreground pro-vides a sense of scale.

Figure 5. Stop 2 borrow pit showing the Upper Cretaceous Middendorf Forma-tion. View is toward the south. Vehicle provides a sense of scale.

8 Swezey et al.


sequences suggests that point bars (which are typical of mean-dering rivers) may have been relatively scarce. Another possibil-ity is that the upper portions of fi ning-upward sequences have been eroded by subsequent channel deposits. Nevertheless, the relatively coarse grain size and the east-dipping cross-bedding are consistent with a proximal fl uvial system fl owing from the Piedmont toward the coast. The clay lens might represent the fi nal stage of accumulation in an abandoned channel, a fl ood-

plain deposit, or some form of post-depositional accumulation of clay within the strata. The sand- and gravel-size clasts of clay are interpreted as rip-up clasts eroded from larger clay beds and/or clay lenses. Most of the clay matrix among the quartz sand grains is thought to be post-depositional. The absence of primary depo-sitional features in the west wall of the pit is attributed to post-depositional pedogenic alteration. More evidence of pedogenic alteration of the upper Middendorf Formation is seen at Stop 5.

Figure 6. East wall of pit at Stop 2 showing cross-bedding in the Upper Cretaceous Middendorf Formation. Twenty-eight-cm- (11-in-) long hammer provides a sense of scale.

Figure 7. South wall of pit at Stop 2 showing clay bed (lens) at the contact be-tween the lower unit with cross- bedding and the upper unit with horizontal planar bedding, Upper Cretaceous Middendorf Formation. Twenty-eight-cm- (11 in-) long hammer provides a sense of scale.



STOP 3: Overlook of the “Sandhills” (Pinehurst Formation) on Wildlife Drive within the Carolina Sandhills National Wildlife Refuge in Middendorf Quadrangle (N 34° 31′ 33.30″, W 80° 13′ 39.89″)

Stop 3 is a parking area on the east side of Wildlife Drive within the refuge. This site provides a good location to observe the “sandhills” that cap the rolling landscape in both the Midden-dorf and Patrick quadrangles, and to present data derived from geologic mapping, LiDAR, ground-penetrating radar (GPR), and

optically stimulated luminescence (OSL). These observations and data provide an informed framework for interpreting the ori-gin and signifi cance of the “sandhills.”

Note the sign that displays information about the red- cockaded woodpecker (Fig. 9). More than 30 plant and animal species associated with the longleaf pine ecosystem are listed as threatened or endangered (Askins, 2010). In ways that are still poorly understood, many of these species require interactions with the longleaf pine and with frequent low-intensity fi res. The most famous endangered species of the refuge is probably the

Figure 8. West wall of pit at Stop 2 showing Upper Cretaceous Middendorf Formation without evidence of obvi-ous primary sedimentary structures. Twenty-eight-cm- (11-in-) long hammer provides a sense of scale.

Figure 9. View of “sandhills” landscape (Quaternary Pinehurst Formation) at Stop 3. Vehicle provides a sense of scale.

10 Swezey et al.


red-cockaded woodpecker (Picoides borealis), which prefers to excavate nesting and roosting cavities in living trees of longleaf pine that are 80–120 yr old (Askins, 2010). The loss of longleaf pine habitat has corresponded with a rapid decline in the red-cockaded woodpecker population, and in 1970 the red-cockaded woodpecker was listed as an endangered species. Within the Car-olina Sandhills National Wildlife Refuge, a white band has been painted around trees that have been identifi ed as having nesting or roosting cavities for these woodpeckers (Askins, 2010).

Stop 3 provides an excellent view of the “sandhills” that overlie the Cretaceous Middendorf Formation. The “sandhills” are a surfi cial unit of unconsolidated sand that is present across all of the Middendorf quadrangle and the Patrick quadrangle, except along the modern creek fl oodplains and the lowest terrace above some of the modern fl oodplains (seen at Stop 10). At most locations, this unconsolidated sand is <1.5-m- (4.9-ft-) thick and does not display any characteristic morphological expression. Images derived from LiDAR data, however, reveal that in areas of higher elevation (such as Stop 3), the sand forms subdued hills of up to 6 m (19.7 ft) relief with steeper sides on the east and southeast (Fig. 10). In these areas, the total thickness of the sand can be more than 6 m (19.7 ft).

The unconsolidated surfi cial sediment of the “sandhills” is composed of grayish-orange (10YR 7/4) sand and muddy sand. Visual inspection of samples in the fi eld indicated that most of

the sediment is of medium sand (upper) to coarse sand (lower) size, using the sediment size terminology of Wentworth (1922) and Folk (1954, 1980). Sieving analyses using mesh sizes at 0.5 phi increments (following Folk and Ward, 1957; Folk, 1966) revealed grain sizes ranging from fi ne sand (lower) to coarse sand (lower), with the most frequently occurring grain size being medium sand (upper) to coarse sand (lower), which is 1.5–0.74 phi (0.35–0.59 mm). Sediment sorting values (sφ) calculated from cumulative percent curves using the sorting formula of Folk and Ward (1957) ranged from 0.76 to 1.65 (moderately sorted to poorly sorted). The sand-sized grains consist predominantly of quartz with some mica and opaque minerals. Most of the quartz grains of medium sand size and coarser were subrounded to sub-angular, ranging from high sphericity to low sphericity (using the sphericity and roundness terminology of Powers, 1953). Textural maturity of individual samples ranged from immature to subma-ture, using the sediment textural maturity terminology of Folk (1951, 1954).

Outcrops of the unconsolidated sand do not display primary sedimentary structures, although some structures are visible in GPR data that was obtained by Kerby M. Dobbs (Fitzwater et al., 2014b). Several GPR traverses across the sandhills, for example, have revealed 2–5-m-thick sets of southeast-dipping cross-bedding at depths below 2 m (6.6 ft) (Fig. 11). In most examples, the dip direction of the cross-bedding is consistent with the dip of the

ELEVATION221 m

17 m50 m

100 m

150 m

200 m

0 1 mile

0 1 km

North

3 2

Figure 10. Detail of shaded relief map shown in Figure 3. This detailed image shows the subdued hills of the “sandhills,” with steeper sides on the east and southeast sides of the hills. White circles and adjacent numbers (2, 3) denote fi eld trip stops. The location of Figure 10 is shown in Figure 3.



SoutheastNorthwest

0

4

8

12 m

0 10 20 30 40 50 60 m

Pine

hurs

tFo

rmat

ion

Mid

dend

orf

Form

atio

n

Pinehurst Formation

Middendorf Formation

Thin white lines show the trace of southeast-dipping cross-beddingin the lower Pinehurst Formation

Figure 11. Ground-penetrating radar (GPR) traverse over one of the sandhills in the Patrick quadrangle, Chesterfi eld County, South Carolina. Location of traverse is shown in Figure 3.

steepest side of the sandhill. The GPR data also show that there is considerable relief on the unconformity that caps the Cretaceous Middendorf Formation. For example, up to 5 m (16.4 ft) of relief is present on the unconformity in this portion of the refuge.

The unit of unconsolidated surfi cial sand within the Carolina Sandhills is mapped as the Quaternary Pinehurst Formation. Con-ley (1962) gave the name Pinehurst Formation to a 3–46-m- (10–150-ft-) thick unit of non-fossiliferous sand and gravel on top of the higher coastal plain hills in Moore County, North Carolina. Bartlett (1967) later examined the Pinehurst type section and delineated a lower unit of sand and clay that he mapped as the Cretaceous Mid-dendorf Formation, which is capped by an unconformity, above which lies loose sand. Bartlett (1967) restricted the term Pinehurst Formation to the loose sand that overlies this unconformity. A sub-sequent summary by Nystrom et al. (1991) stated that widespread deposits of sand are dispersed discontinuously across the upper Coastal Plain from northern Aiken County (South Carolina) north-eastward to the North Carolina state line, and that these deposits are the southwestern continuation of the Pinehurst Formation as rede-fi ned by Bartlett (1967). This unit of sand overlies an unconformity with signifi cant local relief.

OSL dating, which provides an age of the last time that a particular quartz grain was exposed to sunlight, has provided a numerical chronology for the sand. Using this technique, 11 samples of the Pinehurst Formation in the Middendorf and Pat-rick quadrangles have yielded mean OSL ages ranging from ca. 73–19 thousand years (ka) ago and 6 samples have yielded mean OSL ages from ca. 12–8 ka (Swezey et al., 2016). Four-teen of these ages were determined in 2012–2014 by Shannon A. Mahan (USGS, Denver, Colorado), and three of these ages were determined in 2015 by George A. Brook (University of Georgia, Athens, Georgia). Most of these 17 OSL ages are within

the range of 73–19 ka, coincident with the last glaciation in the Northern Hemisphere. Additional details and the signifi cance of these OSL ages are presented and discussed at Stop 9.

In Chesterfi eld County, the unconsolidated surfi cial sand that is mapped as the Quaternary Pinehurst Formation is inter-preted as eolian sand sheets and dunes derived from the immedi-ately underlying Cretaceous sand and sandstone, and stabilized by vegetation under prevailing climate conditions. This interpre-tation is supported by the OSL ages, the sandhill locations, the sandhill morphology as visible in LiDAR imagery, and the GPR data. Furthermore, the OSL ages from the Carolina Sandhills are coincident with other OSL ages from known eolian deposits and features elsewhere in the southeastern United States (e.g., Marke-wich and Markewich, 1994; Ivester et al., 2001; Swezey et al., 2013; Moore et al., 2014).

These new data eliminate several alternative interpretations that have been suggested for the surfi cial deposits in the Caro-lina Sandhills. For example, the OSL ages rule out the possibility of the sand being beach deposits, because sea level was likely to have been well below the elevation of the Sandhills during the time of the last glaciation. The OSL ages rule against fl uvial deposits, because the sand blankets most of the landscape and is not spatially associated with obvious Quaternary fl uvial chan-nels. The OSL ages also rule out the possibility of the sand being associated with the Chesapeake Bay impact crater, which formed during the Eocene.

The location of the sand of the Pinehurst Formation puts some constraints on possible sediment sources and rules out some pos-sible depositional environments. As stated above, the sand is not located adjacent to obvious Quaternary fl uvial channels that might have provided a sediment source. However, the sand is located exclusively in areas where sand or sandstone of the Cretaceous

12 Swezey et al.


Middendorf Formation is near the surface (as noted by Cooke, 1936; Ridgeway et al., 1966). Furthermore, the relatively coarse grain size and poor sorting of the sand suggest that the sand of the Pinehurst Formation has not traveled very far from its source. This spatial association of the unconsolidated Quaternary sand with outcrops of the Cretaceous sand and sandstone (and the relatively coarse grain size and poor sorting of the Quaternary sand) strongly suggests that the Quaternary sand is derived from the underlying Cretaceous strata. As noted by Ridgeway et al. (1966), the similarity of grain sizes and heavy mineral suites also supports the interpretation that the Quaternary unconsolidated sand was derived from the underlying Cretaceous sand.

In relatively fl at, topographically high areas, the unconsoli-dated sand of the Pinehurst Formation forms subdued hills of up to 6 m (19.7 ft) relief, with steeper sides on the east and southeast (Fig. 10). The location of the steeper sides of the hills is consis-tent with the eastward dip directions of cross-bedding in GPR data, thus suggesting that the hill morphology is indeed relict depositional topography of bedforms rather than pure erosional topography unrelated to depositional processes.LUNCH: Lake Bee picnic area (N 34° 34′ 41.41″, W 80° 14′ 11.69″).

Three species of carnivorous plants are present within the Car-olina Sandhills National Wildlife Refuge, and the largest of these plants may be seen along the edge of Lake Bee. The perennial yel-low pitcher-plant (Sarracenia fl ava) or “trumpet” plant forms leaf tubes up to 1.0 m (3.3 ft) tall, and grows in moist soils that are poor in nitrogen (Sorrie, 2011). The pitcher-plants compensate for this nitrogen defi ciency by trapping insects in leaf tubes and digesting the insects using chemicals secreted by the leaves.

Various adaptations allow these carnivorous plant species to compete in nutrient-poor soils such as are common in the sandhills and in wetlands. Groundwater seeps and toe-slope springs used to be much more abundant across the Coastal Plain when the longleaf pine and wiregrass savanna were the domi-nant upland vegetation. The more dense foliage in present native forests intercepts and transpires much greater amounts of rain-fall, soil moisture, and shallow groundwater than savannas with widely space trees and open understory (e.g., Riekerk 1983; Win-ston, 1995; Steven and Toner, 2004). In one Coastal Plain site in southern Virginia where oak-pine vegetation was removed and longleaf pine savannas are being restored, the recharge to shallow groundwater systems increased by 30% during years with normal precipitation (McLeod, 2012). The resultant higher-water tables and increased number of groundwater-seepage wetlands, along with the frequent fi res that kill competing vegetation, allow the carnivorous plants to thrive.

STOP 4: Middendorf Outcrops at Lake Bee Picnic Area within the Carolina Sandhills National Wildlife Refuge in Middendorf Quadrangle (N 34° 34′ 41.41″, W 80° 14′ 11.69″)

Stop 4 at the Lake Bee picnic area shows outcrops of yellow-ish-gray (5Y 8/1) to grayish-pink (5R 8/2) sandstone with occa-

sional spots of pale red (5R 6/2). This sandstone forms a distinct facies of the Middendorf Formation. The sandstone consists of moderately sorted medium sand composed primarily of quartz (Fig. 12). Fossils have not been found in this facies of the Mid-dendorf Formation. The red spots are caused by an iron cement, and some of these spots are centered on vugs with diameters of ~0.5 cm or 0.2 in. Primary sedimentary structures are not obvi-ous, but there is a hint at this outcrop, and other nearby outcrops, that the unit consists of relatively horizontal to slightly undula-tory beds that are 0.2–0.6 m (0.7–2.0 ft) thick.

This unit forms distinct ledges that crop out at ~107 m (350 ft) elevation throughout the western portion of the Midden-dorf quadrangle. At a location 0.3–0.6 km (0.2–0.4 mi) down-stream from Lake Bee along the north side of Hemp Branch, this unit forms a very prominent series of ledges, and water seeps from beneath these ledges at times when much of the rest of the wildlife refuge is relatively dry. These ledges provide some of the very few locations where one might attempt to measure strike and dip, and they show that the strata in this area are very near to horizontal. Regional context suggests that there is probably a very slight dip to the east.

The yellowish-gray (5Y 8/1) to grayish-pink (5R 8/2) sand-stone exposed at this stop is interpreted as fl uvial sediments. In the absence of primary sedimentary structures, however, it is dif-fi cult to situate this facies more precisely within the fl uvial sys-tem context. Nevertheless, the facies is widespread in the western portion of the Middendorf quadrangle, and it occurs at a very predictable elevation in the upper portion of the Middendorf For-mation. This facies is also found at elevations of 64–85 m (210–280 ft) in the Patrick quadrangle.

STOP 5: Outcrop of Middendorf Formation (with Plinthite) on Wire Road 4 within the Carolina Sandhills National Wildlife Refuge 3.7 km (2.3 mi) West of the Middendorf Quadrangle (N 34° 29′ 40.72″, W 80° 16′ 51.93″)

Stop 5 is located on the south side of Wire Road 4, ~3.7 km (2.3 mi) west of the Middendorf quadrangle and 2.4 km (1.5 mi) east of Highway 151, within the southwestern portion of the ref-uge. This stop shows an excellent exposure of the uppermost por-tion of the Cretaceous Middendorf Formation where it is deeply weathered (Fig. 13). At this stop, we discuss the characteristics of plinthite-bearing soil profi les in the Coastal Plain and their geo-morphic and stratigraphic signifi cance. For this stop, the color nomenclature used is from the Munsell Soil Color Chart (Mun-sell Color, 2000).

Several distinct beds are visible in this outcrop of the Mid-dendorf Formation. Most of these beds were originally sandy, but have been greatly modifi ed by pedogenic processes. Primary sed-imentary structures are not obvious because of the overprinting of pedogenic clay and remobilized iron in the soil. The network of irregular lines and blobs marked by tan, yellow, orange, and red is called reticulate mottling. At this site, the reticulate mot-tling dominates a meter-thick zone across the entire exposure.



Within this zone are strong brown (7.5YR 5/6) masses of oxi-dized iron, and light brownish-gray (10YR 6/2) iron-depleted masses. Beneath this zone for at least another meter, the reticu-late mottling becomes less dominant but is still common. Above the zone of reticulate mottling, the Middendorf strata are a more uniform red (10R 5/8) sandy loam with a few depleted mottles, many of which resemble root paths or animal burrows.

According to Soil Survey Staff (2006), reticulate mottling forms by the dissolution and then precipitation of iron when the soil experiences alternating reducing and oxidizing conditions,

such as occurs in the zone of a fl uctuating water table. Because the water table is no longer close to the surface at this well-drained site, the iron masses and depletions are interpreted as relict redoximorphic features (Morton, 1995).

Plinthite masses are iron-rich mixtures of kaolinitic clay with quartz, and these masses commonly have well-developed reticulate mottling (Soil Survey Staff, 2006). What is now termed “plinthite” was called “laterite” in some older soil literature (Schaetzl and Thompson, 2015). In plinthite, when the soil is moist, the concentrations of iron particles can be soft enough

Figure 12. Yellowish-gray to grayish-pink sandstone of the Cretaceous Mid-dendorf Formation at Stop 4 near Lake Bee. Twenty-eight-cm- (11-in.-) long hammer provides a sense of scale.

Figure 13. Plinthite at the top of the Cre-taceous Middendorf Formation at Stop 5. Hammer provides a sense of scale.

14 Swezey et al.


to be sliced with a shovel. If plinthite dries and hardens, then the iron particles cannot be re-softened, and the resulting larger hardened masses are called ironstone (Soil Survey Staff, 2006) or petroplinthite (Eze et al., 2014). If eroded and transported from an outcrop, then plinthite and/or ironstone particles and masses may survive transport by slope processes, currents, and/or waves, and may be found as reworked clasts in younger sediment accu-mulations (e.g., Whittecar et al., 1993, 1994).

At the Stop 5 outcrop, the plinthite and reticulate mottling are interpreted as pedogenic features that formed on the uncon-formity at the top of the Middendorf Formation. At many sites throughout the Middendorf and Patrick quadrangles, the upper-most Middendorf Formation and the unconformity that caps it show evidence of a substantial pedogenic overprint, but typically only the lower portions of deeply weathered soil profi les with occasional redoximorphic features have been preserved. Other indicators of intense and/or long-lasting chemical weathering occur throughout the formation (e.g., clay pseudomorphs or “ghosts” of feldspar sand, clasts and beds of mud that has altered to kaolin). As mentioned previously, GPR data show evidence of substantial relief on this unconformity (at least 5 m or 16.4 ft locally), suggesting that much erosion has taken place in addition to the pedogenic modifi cations.

One interpretation of these features is that they developed during periods of intense and long-lasting weathering under humid tropical conditions. Plinthites are common in many oxi-sols that form today in humid tropical and subtropical environ-ments within poorly and somewhat poorly drained soils (Eze et al., 2014; Schaetzl and Thompson 2015). High temperatures, a landscape position with abundant moisture, and a fl uctuating water table seem to be necessary conditions for their formation (e.g., Osher and Buol, 1998), and the availability of iron aids the process considerably (e.g., Smith, 2007). Thus the presence of plinthite may indicate that the landscape at the site used to be a lowland in a tropical or subtropical climate. The common occurrence of plinthitic soils on dissected uplands (such as here in the Carolina Sandhills) would indicate a geomorphic history of stream incision and topographic inversion that changed lowlands into an upland plateau (Eze et al., 2014).

In the southeastern United States, more than 50 soil series contain plinthitic features (Shaw et al., 2011). These plinthitic soils are present in high-elevation Coastal Plain settings, and they are reported only from Pliocene- and Miocene-age surfaces (e.g., Cady and Daniels, 1968). In southeastern Virginia, Miocene deposits along the Fall Zone contained in situ plinthite concentra-tions, but in the Pliocene surfaces, the ironstone particles formed from plinthite are rounded clasts that have been reworked within shore-zone sediments (Whittecar et al., 1994).

Although it is possible that some of the features of chemical weathering in the Middendorf Formation date from periods as old as the Cretaceous, it is generally very diffi cult in the southeastern United States to demonstrate that an existing landscape surface and its related weathering phenomena are older than the Miocene (e.g., Whittecar et al., 2016). Stream incision and slope processes

have removed kilometers of landscape prior to the Miocene, and some regions of the Piedmont may have been eroded by marine transgressions during the Miocene (e.g., Weems and Edwards, 2007). Even so, climates in the Carolinas during some parts of the Miocene were notably warmer than present (e.g., Poag and Sevon, 1989), and areas even as far north as Delaware may have been subjected to tropical to subtropical conditions for millions of years (e.g., Ward, 1998). Thus, deep and intense weathering conditions during the Miocene may have developed long enough for existing relict landscapes to form some of the plinthite and reticulate mottling seen in outcrops at this stop.

In summary, plinthite in the inner coastal plain is a clear indication of ancient soils that formed at sites with high fl uctuat-ing water tables. Such plinthite may be the result of deep tropi-cal weathering that occurred during pre-Pliocene periods. The plinthite could also have formed during a warm but cooler-than- tropical climate, if given enough time and a very stable landscape.

STOP 6: Patrick Core Pit 305 m (1000 ft) East of Ruby–Hartsville Road within the Sand Hills State Forest in Middendorf Quadrangle (N 34° 33′ 10.98″, W 80° 07′ 40.76″)

Stop 6 is located 305 m (1000 ft) east of Ruby–Hartsville Road within the Sand Hills State Forest. As described by Askins (2010), this property was once part of the Carolina Sandhills National Wildlife Refuge. In 1991, however, a portion of the Car-olina Sandhills refuge was transferred from the Federal govern-ment to the South Carolina Forestry Commission, thus forming the Sand Hills State Forest.

Stop 6 is a borrow pit that provides an excellent exposure of the unconformity between the Upper Cretaceous Midden-dorf Formation and the overlying Quaternary Pinehurst Forma-tion (Fig. 14). This pit is also the location where the USGS drilled an 85.3-m- (280-ft-) deep core in March 2014. This pit is located 0.3 km (0.2 mi) northeast of the type section of the Middendorf Formation.

This stop is an excellent place to discuss stratigraphic nomen-clature. Darton (1896) described a several-hundred-ft-thick unit of Cretaceous sand, sandstone, and clay extending from Colum-bia to Cheraw, and he designated this unit as the Potomac Group after similar strata described and named by McGee (1886) from outcrops along the Potomac River near Washington, D.C. Sloan (1904, 1907), however, indicated that a precise equivalent of the Potomac strata is not present in South Carolina, and he subdi-vided the approximately equivalent South Carolina strata into (1) a lower unit of white and yellow sand with clay beds and kaolin (collectively named the Hamburg Beds after the now-abandoned community of Hamburg in Aiken County) and (2) an upper unit of white, yellow, orange, pink, and purple sand with clay beds and kaolin (collectively named the Middendorf Beds after the community of Middendorf in Chesterfi eld County). In subse-quent studies, Berry (1910, 1914) used the term “Middendorf arkose member of the Black Creek Formation” in a study of Cre-taceous plant fossils near the community of Middendorf. Cooke



(1926) used the term “Middendorf Formation,” and he stated that this formation should include both the “Hamburg Beds” and the “Middendorf Beds” of Sloan (1904, 1907). Cooke (1936) desig-nated a specifi c type locality of the Middendorf Formation as an outcrop on the railroad line at the crossing of the Hartsville–Ruby Road, 2 mi northeast of Middendorf, in Chesterfi eld County, South Carolina (the type locality is 0.3 km [0.2 mi] southwest of Stop 6). Cooke (1936) also stated that the Middendorf Formation in South Carolina extends across Georgia and is identical to the Tuscaloosa Formation, which is an older stratigraphic term from Alabama (Smith and Johnson, 1887) that should take precedence. Swift and Heron (1969), however, indicated that the term “Tus-caloosa Formation” has been used in the Carolinas to describe two lithologically distinct bodies of strata that are separated by an unconformity. The lower strata consist of numerous laterally persistent units of gravelly sand grading up to sand and muddy sand exposed along the Cape Fear River and other areas near Fayetteville, North Carolina. They suggested that the term “Cape Fear Formation” be applied to these strata, following nomencla-ture proposed by Stephenson (1907, 1909, 1912). In contrast, the upper strata consist of heterogeneous units of pebbly sand, clean sand, muddy sand, and clay beds that are widespread in the Carolinas. Swift and Heron suggested that the name Middendorf Formation be applied to these strata. Indeed, many publications continued to use the term Middendorf Formation (e.g., Heron, 1958a, 1958b, 1959; Ridgeway et al., 1966; Swift and Heron, 1967, 1969; Howell and Zupan, 1974), and eventually the U.S. Geological Survey reinstated the stratigraphic term “Middendorf Formation” in South Carolina and North Carolina (Cohee and

Wright, 1976). Since then, Woollen and Colquhoun (1977) used the term Middendorf Formation in Chesterfi eld and Darlington Counties, Owens (1989) used the term Middendorf Formation on his 1:250,000-scale geologic map of northeastern South Carolina and southeastern North Carolina, and Prowell et al. (2003) used the term Middendorf Formation in the vicinity of the Middendorf type locality. Recent geologic maps by Fitzwater, Swezey, and Whittecar of the 1:24,000-scale Middendorf quadrangle and the Patrick quadrangle have also used the stratigraphic term of Mid-dendorf Formation.

In conjunction with the geologic mapping of Middendorf and Patrick quadrangles, the USGS drilled a core in March 2014 at Stop 6 (Fig. 15). This core (Patrick core, CTF-320) started at a ground elevation of 112.8 m (370 ft) above sea level (ASL) and reached 85.3 m (280 ft) total depth. In this core, the strata are predominantly sandstone to pebbly sandstone (sand, where poorly lithifi ed), except at 38.1–26.8 m (125–87.9 ft) depth where clay is the predominant lithology. This thick interval of clay can be seen in outcrop at a kaolinite mine off Bullard Ford Road (south of Little Beaverdam Branch) in the southeast por-tion of the Middendorf quadrangle. Fining-upward sequences and cross-bedding are visible in some sandstone units of the Patrick core. Other lithologies include 0.2–5.0-m- (0.7–16.4-ft-) thick beds of clay to silty clay, <1.5-m- (4.9-ft-) thick units of alternating laminations of sandstone (sand, where poorly lithifi ed) and clay, and <1.0-m- (3.3-ft-) thick units of pebbly sandstone to gravel. An assemblage of coarse to very fi ne sand with disrupted bedding, mottled texture, and downward taper-ing vertical structures (interpreted as a paleosol) is present in

Figure 14. Stop 6 outcrop showing the Upper Cretaceous Middendorf Forma-tion, capped by unconformity, which is overlain by Quaternary Pinehurst For-mation. Shovel and hoe provide a sense of scale.

16 Swezey et al.


Figure 15. Diagram of the Patrick and Cheraw cores (Cretaceous Middendorf Formation), Chesterfi eld County, South Carolina. In this fi gure, both cores are hung relative to sea level. The distance between the two cores is ~25 km (15.5 mi). See Figure 2 for core locations.



the Patrick core at 85.3–81.7 m (280.0–268.0 ft) depth or 27.5–31.1 m (90.2–102.0 ft) ASL.

A similar core was drilled by the USGS in July 1995 at Cheraw State Park, which is located ~25 km (15.5 mi) northeast of the Patrick core. This core (Cheraw core, CTF-081) started at a ground elevation of 58.8 m (193 ft) ASL, reached the contact of the Middendorf Formation over Paleozoic schist (meta-siltstone) at 66.4 m (218 ft) depth, and reached a total depth of 74.4 m (244 ft) in the schist. As with the Patrick core, the strata in the Cheraw core are predominantly sandstone to pebbly sandstone. Fining-upward sequences and cross-bedding are visible in some sandstone units, and other lithologies include <1.5-m- (4.9-ft-) thick units of clay to silty clay, and <1.5-m- (4.9-ft-) thick units of alternating sand and clay laminations. An assemblage of coarse to very fi ne sand with disrupted bedding, mottled texture, and downward tapering vertical structures (interpreted as a paleosol) is present in the Cheraw core at 38.1–36.3 m (125.0–119.1 ft) depth or 20.7–22.5 m (67.9–73.8 ft) ASL. Pebbles of smoky quartz and milky quartz up to 5 cm (2 in) diameter are present at the base of the Middendorf Formation in the Cheraw core (Fig. 16).

The two cores do not reveal signifi cant changes in lithology such that separate stratigraphic terms might be warranted, and thus the term “Middendorf Formation” is applied to all of the cored intervals above the Paleozoic schist. However, grain size is generally coarser in the lower part of the section. For example, the predominant lithologies in the lower Middendorf Formation (below the paleosols) are coarse sand to gravel, whereas the pre-dominant lithologies in the upper Middendorf Formation (above the paleosols) are medium sand and clay.

The Middendorf Formation is interpreted to be fl uvial sedi-ments derived from highlands to the west including the uplifted margins of Mesozoic rift basins and/or the Appalachian Moun-tains. A diagram of facies associated with a macrotidal estu-ary helps to portray this interpretation (Fig. 17). The sandy and gravelly units of the Middendorf Formation are interpreted as primarily channel deposits, whereas the thick clay units are inter-preted as fl oodplain deposits and abandoned channel deposits. In a sequence stratigraphic context, the Middendorf Formation is interpreted as fl uvial sediments that accumulated during base-level fall (lowstand) and subsequent early transgression. The ini-tial base-level fall resulted in relatively coarse fl uvial sediments prograding into the basin across the unconformity on Paleozoic schist. As the system changed from lowstand conditions to early transgression, fl uvial sediment grain size became relatively fi ner and a greater proportion of fl oodplain muds was preserved (Fig. 18).

The fl uvial interpretation of the Middendorf Formation is consistent with the leaf fossils identifi ed by Berry (1910, 1914) at the Middendorf type section, and with the pieces of petrifi ed wood found in nearby outcrops. Furthermore, silty clay at 65.5 m (215.2 ft) depth in the Cheraw core has yielded Late Cretaceous palynomorphs including abundant fern spores (Cicatricosi-sporites, Gleicheniidites, Foveotriletes), as well as gymnosperm

Figure 16. Lowermost portion of the Cheraw core (CTF-081), show-ing unconformity between Paleozoic schist (meta-siltstone) and over-lying Cretaceous Middendorf Formation. The numbers on the wooden blocks denote depth from surface in feet. The unconformity is placed at 66.4 m (218 ft) depth, and multi-colored quartz pebbles are present on this unconformity, just below the 217.5 ft depth marker.

pollen (Taxodiaceae and Pinuspollenites), angiosperm pollen (Momipites), and freshwater algae (Schizosporis). In modern environments, Taxodiaceae does not tolerate high salinity, and is typically found on intermittently fl ooded, poorly drained fl ood-plains (i.e., additional evidence suggesting a fl uvial rather than estuarine or marine deposit). These palynomorphs were iden-tifi ed by Christopher Bernhardt (USGS), and are reported in Swezey et al. (2015).Drive from McBee to Hartsville, going over Orangeburg Scarp.Spend the night in Hartsville.

DAY 2 (3 APRIL 2016)Drive from Hartsville to McBee (going over Orangeburg Scarp) and then to the Sand Hills State Forest near the town of Patrick.

On this second day of the fi eld trip, we drive northwest from Hartsville to McBee along Highway 151. As we cross the bound-ary from Darlington County to Chesterfi eld County, we go up the Orangeburg Scarp (Fig. 19), an east-facing scarp where the

18 Swezey et al.


topography changes abruptly from ~60–80 m (197–262 ft) eleva-tion to 100–120 m (328–394 ft) elevation. This geomorphic fea-ture was named for a site at the City of Orangeburg in Orangeburg County of South Carolina (Colquhoun, 1962), and is interpreted as a shoreline terrace formed by wave erosion at a time of high sea level during the middle Pliocene (Dowsett and Cronin, 1990).

After driving northwest up the Orangeburg Scarp and onto the plateau of Cretaceous strata, there is a fi eld trip stop at a major north-trending drainage divide, followed by a stop at Sugarloaf Mountain to look at various facies of the Cretaceous Middendorf Formation, and then a stop along Isaac Road to look at an exceptional exposure of the Quaternary Pinehurst Formation. After lunch back at Sugarloaf Mountain, the fi eld trip stops on private property to look at terraces along Juniper Creek and to examine contacts between these terraces and sand of the Pinehurst Formation.

STOP 7: Drainage Divide on Road between Scotch Road and Sugarloaf Mountain within the Sand Hills State Forest in Middendorf Quadrangle (N 34° 35′ 08.10″, W 80° 08′ 10.44″)

Stop 7 is an overlook that shows the drainage divide on the road to Sugarloaf Mountain within the Sand Hills State Forest in the Middendorf quadrangle (Fig. 20). This drainage divide is a north-trending escarpment incised by headwater streams. The divide separates the predominantly south-fl owing drainages asso-ciated with Black Creek from the predominantly east-fl owing drainages associated with Juniper Creek and similar tributaries of the Pee Dee River (Fig. 2). The recognition of this escarpment is an excellent example of how LiDAR data are transforming our ability to resolve topographic features in the Coastal Plain. On the ground, this drainage divide appears to be a relatively subtle escarpment of low relief (Fig. 20), whereas in the LiDAR

Figure 17. Facies associated with a macrotidal estuary, such as the Gironde estuary in southwestern France (Allen, 1991). The Cretaceous Middendorf Formation is analogous to the upstream (“fl uvial”) portion of the diagram.



imagery, this escarpment appears as a prominent feature charac-terized by broad arcuate embayments carved by stream headwa-ters on its eastern side (Fig. 3).

The origin of the marked asymmetry across this divide may stem from several factors, including Quaternary climate changes, erosion-resistant strata, and long-term migration of the drainage divide. The watersheds on both sides of this divide are tributaries to the Pee Dee/Yadkin drainage, a large fl uvial net-work that drains part of the Blue Ridge and Piedmont provinces in the Carolinas and then crosses the Coastal Plain province. During times of sea-level lowstand, episodes of headward ero-sion may have occurred along tributaries such as Black Creek and Juniper Creek that join the Pee Dee River in the Coastal Plain province. As discussed in greater detail at Stop 10, the sequences of strath terraces along Juniper Creek indicate that a modest amount of such incision occurred episodically during the Pleistocene. Whittecar et al. (2013) suggested that such incision events along streams could infl uence landscape stability along the drainage divide. However, no regional-scale mapping and

correlation of terraces through these fl uvial systems yet exists, and so it is unclear whether the divide asymmetry is infl uenced more by Quaternary changes in climate and/or sea level, or whether longer-term factors are more important.

Another reason for the drainage divide asymmetry could be resistant beds within the Middendorf Formation. Morphologi-cally, the embayments along the divide resemble those described by Dunne (1990) that form by groundwater sapping and fi rst-order stream erosion below resistant cliff-forming strata. It is rea-sonable to ponder if the scarp at the drainage divide is supported in part by iron-cemented sandstone (such as that seen at Sugar-loaf Mountain; Stop 8) or by weakly cemented sandstone (such as that seen at Stop 2). All evidence to date from mapping, how-ever, suggests that moderate or strong cementation only occurs at the scale of individual outcrops, with one exception being trace-able for a kilometer (Stop 4). No examples of such cementation over long distances are known from the slopes along the drainage divide. Furthermore, strong cementation of the Middendorf For-mation is not present at depth in the Patrick core or Cheraw core.

Figure 18. Model of fl uvial sequence stratigraphy; modifi ed from Wright and Marriott (1993).

20 Swezey et al.


The most plausible explanation for the drainage divide asymmetry is long-term migration of the divide caused by differ-ent slopes of the entire watershed. Streams fl owing west through the Black Creek system travel ~86 km (53 mi) to join the Pee Dee River at 14 m (46 ft) elevation (0.16 m/km or 0.87 ft/mi), whereas Juniper Creek tributaries travel ~28 km (17 mi) to join the Pee Dee at 26 m (85 ft) elevation (0.92 m/km or 5 ft/mi). The steeper watershed gradient of the Juniper Creek tributaries allows them to be more erosive and to drive the divide westward over time. Willett et al. (2014) developed a parameter, χ, that allows a determination of the degree of disequilibrium that exists between streams across divides, and the direction in which the divide

Figure 19. The Orangeburg Scarp on Highway 151 near the border between Darlington County and Chesterfi eld County, South Carolina. View is to the northwest.

Figure 20. East-facing arcuate embay-ment along north-trending drainage divide at Stop 7. View is to the south. The white line traces the edge of the es-carpment, and a change from relatively higher ground to topographically lower ground on the escarpment face. This es-carpment is very diffi cult to portray in a photograph, and it may be helpful to re-fer to the location of Stop 7 and the east-facing escarpment as shown in Figure 3.

should migrate over geologic time. χ maps suggest that the Black Creek–Juniper Creek divide in the study area may be in a state of disequilibrium, albeit a relatively modest one when compared to divides between larger drainage basins in the Coastal Plain and Piedmont provinces of the Carolinas. If more detailed analyses of χ across this region confi rm the disequilibrium, then this con-fi rmation would support the observations that overall topography across most of the Patrick quadrangle is younger than the higher elevation topography within the Middendorf quadrangle. The unusually thick, deeply weathered soils on the broad interfl uvial regions within the Middendorf quadrangle (see Stop 5) would also support this disequilibrium hypothesis.

STOP 8: Sugarloaf Mountain within the Sand Hills State Forest in Patrick Quadrangle (N 34° 35′ 15.24″, W 80° 07′ 04.73″)

Stop 8 is located at Sugarloaf Mountain, a prominent out-crop of the Cretaceous Middendorf Formation in the Patrick quadrangle (Figs. 21 and 22). This outcrop has been known for a long time, and a measured section of the strata here was pub-lished by Sloan (1904).

Near the base of Sugarloaf Mountain, beds or lenses of clay form a distinct facies of the Middendorf Formation. Such clay beds are more common in the upper Middendorf Formation, and similar clay beds or lenses were seen at the pit at Stop 2 and in the Patrick core (CTF-320) at Stop 6. These clay lenses and beds consist predominantly of kaolinite. Although some kaolinite in the Carolina Sandhills is likely to be of diagenetic origin (e.g., kaolin matrix around quartz sand grains), most beds and lenses of kaolinite are interpreted as fl oodplain deposits or the fi nal stage of sediment accumulation in an abandoned channel. It is possible



that these beds and lenses were composed initially of a different clay mineral, and have subsequently been altered to kaolinite.

At Stop 8, the most prominent facies of the Middendorf Formation is that of iron-cemented cross-bedded sandstone to pebbly sandstone. Blocks of this facies form the top of Sug-arloaf Mountain, and other blocks are scattered around the base of the mountain where they appear to have rolled down from above (Fig. 21). This facies is found at several locations throughout the Middendorf and Patrick quadrangles, but has not been identifi ed in the subsurface. Furthermore, the iron cemen-tation is restricted to relatively coarse-grained strata. The cross-bedded sandstone to pebbly sandstone is interpreted as fl uvial sediments (primarily channel deposits, as depicted in Fig. 17), and the iron is attributed to some form of diagenetic shallow groundwater circulation. After the sandstone is cemented by iron, the sandstone is more resistant to erosion and forms topo-graphic highs across the landscape.

STOP 9: Isaac Road Outcrop of Pinehurst Formation in Patrick Quadrangle (N 34° 37′ 17.67″, W 80° 03′ 34.35″)

Stop 9 is located on the east side of Isaac Road, and one wonders if the name of this road has a specifi c association with the longleaf pine. When the sap of the longleaf pine was distilled, the resulting product was called rosin (Earley, 2004). There were 12 different grades of rosin, and “Isaac” was the name of a par-ticular grade.

Stop 9 is an exceptional outcrop of the Quaternary Pine-hurst Formation (Fig. 23). As with most outcrops of this unit, primary sedimentary structures are not visible but there is evi-dence of bioturbation by vegetation, as well as pedogenic fea-tures including soil lamellae and argillic horizons. Specifi cally,

this site shows a buried Bt horizon of a paleosol in soil profi les of the Candor association (Morton, 1995) within the Pinehurst Formation. At this stop, we discuss these pedogenic features, the signifi cance of the OSL ages obtained, and the potential use of soil maps in the development of geologic maps. For this stop, the color nomenclature used is from the Munsell Soil Color Chart (Munsell Color, 2000).

The Quaternary Pinehurst Formation is surfi cial sand with weak to moderate weathering (i.e., soil lamellae to brown Bt horizons) that drapes over most of the uplands and terraces in the Middendorf and Patrick quadrangles. The thickness of this sand unit ranges from tens of centimeters to several meters. How-ever, on the recent geologic map of the Patrick quadrangle (to be published by the South Carolina Geological Survey), the Pine-hurst Formation is portrayed only where the sand thickness is greater than 2 m (6.6. ft). Although it could be a considerable challenge to defi ne an approximate map boundary at 2 m (6.6 ft) sand thickness, the defi nitions of two of the soil series mapped in this county by Morton (1995)—the Alpin sand and the Condor sand—meet this geologic map criteria (Table 1). Other soils in the area also have notable sand mantles (Table 1), but the sand thickness of these other soils is generally <2 m (6.6 ft). For exam-ple, surfi cial sand in the Troupe soil is up to 2 m (6.6. ft) thick, and surfi cial sand in the Ailey sand and Vaucluse loamy sand is 0.5–1 m (1.6–3.3 ft) thick (Morton, 1995).

Buried surfaces and soil horizons within their characteristic profi les are present in most of the soil series of the Carolina Sand-hills. The Troupe, Ailey, and Vaucluse soil series each contain subsoil horizons with reddish colors (e.g., 2.5YR) that are usu-ally found in deeply weathered deposits of the Cretaceous Mid-dendorf Formation. When exposed in outcrop, these deep hori-zons often contain relict redoximorphic features and structures

Figure 21. Block of iron-cemented sandstone that has come down from Sugarloaf Mountain at Stop 8. Sugar-loaf Mountain is located on the left side of the photograph. Vehicle provides a sense of scale.

22 Swezey et al.


that could indicate the presence of a truncated paleosol. The Alpin soil has no buried soils within the uppermost 2 m (6.6 m), but the Candor sand contains a brown Bt horizon on the top of a buried sand bed. Overlying this argillic horizon is a full A-E-Bt-C profi le that has formed in the surfi cial sand sheet or dune sand.

The brown colors in these horizons (e.g., 10YR) suggest that they developed during the Quaternary.

Other soil types occur in only a very few places (<10%) on the uplands (Morton, 1995). The Noboco loamy sand (oxy-aquic paleudult), for example, with its strongly oxidized nodules

Figure 22. Measured section by Sloan (1904, p. 107–108) at Sugarloaf Mountain. Sloan (1904) also mentioned that 6.1 m (20 ft) of “drab clay” was exposed in a pit at the base of Sugarloaf Mountain. The top of Sugarloaf Mountain is 156.4 m (512 ft) above sea level, and thus this section is stratigraphically above all of the units in the Patrick core.



and reduced masses in its upper Bt horizon, appears to be a soil formed where the Middendorf sediments now have no overlying Quaternary sediments. Terrace deposits (discussed later at Stop 10) have their own suite of soil series that formed on sand and mud deposited by streams.

The following three mean OSL ages were obtained from the Pinehurst Formation at this site: 67.1 ± 5.5 ka, 56.1 ± 4.9 ka, and 61.3 ± 2.8 ka. These three ages were determined in 2014 by Shannon Mahan (USGS). As mentioned at Stop 3, most of the OSL ages from the Pinehurst Formation range from ca. 73–19 ka, coincident with glacial conditions in the Northern Hemisphere. It is important to remember, however, that the individual OSL ages indicate only the last time that a particular sample was exposed to sunlight, and not the total time of sediment mobilization. No OSL ages from the Carolina Sandhills are younger than 8 ka, and thus it appears that since this date, the eolian sand sheets and dunes have remained stabilized by vegetation and subjected to pedogenic processes.

In a broader context, the interpretation of the Carolina Sand-hills as eolian sand sheets and dunes is consistent with other evi-dence for eolian activity in the southeastern United States during the last glaciation and early deglaciation. Specifi cally, parabolic eolian dunes in river valleys of the Coastal Plain from Georgia through Delaware were active ca. 40–19 ka (Markewich and Markewich, 1994; Ivester et al., 2001; Swezey et al., 2013). Addi-tional evidence of eolian activity during this time comes from Carolina Bays, which are oval depressions on the Coastal Plain interpreted as having formed by eolian defl ation ca. 44–10 ka (Ivester et al., 2002, 2003; Moore et al., 2014).LUNCH: Picnic area at Sugarloaf Mountain (N 34° 35′ 15.24″, W 80° 07′ 04.73″).

STOP 10: Tommy Wilkes Farm in Patrick Quadrangle (N 34° 34′ 46.49″, W 80° 01′ 32.35″)This is private property. Please obtain consent from the owner before proceeding onto the property.

Stop 10 is located on one of the larger terrace complexes in the study area and provides a good location to discuss the relations between Quaternary terrace deposits, the Pinehurst Formation, and pedogenic features. For this stop, the color nomenclature used is from the Munsell Soil Color Chart (Mun-sell Color, 2000).

Throughout the Patrick quadrangle, the following three groups of terraces exist along streams and creeks: (1) low-level terraces that are 0–4.6 m (0–15 ft) above creek level (ACL), for most creeks within the Middendorf and Patrick quadrangles; (2) mid-level terraces that are 4.6–9.1 m (15–30 ft) ACL; and (3) high-level terraces that are >9.1 m (30 ft) ACL. The fact that there are three groups of terraces suggests that there have been at least three episodes of geomorphologic disturbance that have led to terrace formation.

The terrace deposits are <3 m (9.8 ft) thick, and are com-posed of moderately sorted to poorly sorted pebbles, pebbly

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24 Swezey et al.


sand, sand, silt, and/or clay. At many sites, these terrace sedi-ments are arranged in a general fi ning-upward sequence. The pebbles range up to 1.9 cm (0.7 in.) diameter, and are composed of smoky quartz, milky quartz, and clear quartz. Some clasts of light-gray clay up to 2 cm (0.8 in.) diameter are also present. The sand, which ranges in size from fi ne to coarse, is composed of quartz with minor amounts of mica. Terrace sediments that are closer to the modern fl oodplains are typically brown and yellow, whereas higher terrace sediments contain reddish-yellow and brown colors.

At Stop 10 (Wilkes farm site), a backhoe was used to dig two trenches into the sandy hillslope deposits identifi ed through GPR and borehole data (Fig. 24). The trenches were ~18.3 m (60 ft) apart, and located at ~34° 34′ 44.2″ N, 80° 01′ 43.2 ″ W at ~61 m (200 ft) elevation.

The fi rst trench (“Trench 1” in Fig. 24) was 2.5 m (8.2 ft) deep and contained only sand of the Quaternary Pinehurst For-mation. The soil profi le in this trench consisted of a thin O hori-zon, a 0.2-m- (0.7-ft-) thick brown (10YR 5/3) sandy Ap horizon, a 0.3-m- (1.0-ft-) thick dark yellowish-orange (10YR 6/6) sandy E horizon, and a 1.0-m- (3.3-ft-) thick brown (7.5YR 5/8) loamy sand B horizon. The B horizon was separated from the under-lying C horizon by a gradual wavy boundary with visible root structures. The underlying 1-m- (3.3-ft-) thick sandy C horizon had a color of brownish yellow (10YR 6/6). One OSL sample was taken at a depth of 2.5 m (8.2 ft) within the C horizon, and yielded a weighted mean age of 33.8 ± 3.4 ka.

The second trench (“Trench 2” in Fig. 24) was 3 m (9.8 ft) deep and contains sand of the Quaternary Pinehurst Formation (Fig. 25). The soil profi le in this trench consisted of a thin O horizon as well as a 0.4-m- (1.3-ft-) thick brown (10YR 5/3) sandy Ap horizon. Directly below the Ap horizon from 0.4 to 1.9 m (1.3–6.2 ft) depth, there was a white (10YR 8/1) and yel-low (10YR 8/6) sandy C horizon. Within the C horizon and starting at a depth of 0.8 m (2.6 ft) below the surface, there are multiple brownish-yellow (10YR 6/8) soil lamellae that are up to 1 cm (0.4 in) thick in places. This material was sampled at

the base of the C horizon at a depth of 1.9 m (6.2 ft) for OSL dating, and yielded a weighted age of 12.1 ± 2.0 ka. An abrupt boundary separated the C horizon from the underlying deposit, which was a 0.8-m- (2.6-ft-) thick brownish-yellow (10YR 6/8) loamy sand. This loamy sand, which was interpreted as a B

b

horizon, was sampled at a depth of 2.0 m (6.6 ft) for OSL dating, and yielded a weighted age of 29.2 ± 4.9 ka. A wavy boundary separated the B

b horizon from an underlying sandy loam that

contains mottling. This sandy loam was exposed in the bottom 0.3 m (1.9 ft) of the trench.

Geologic mapping throughout the Patrick quadrangle has shown that sand of the Quaternary Pinehurst Formation is pres-ent on the mid-level terraces and the upper-level terraces, but not on the low-level terraces (i.e., terraces that are <4.6 m [15.1 ft] ACL) or on the modern fl oodplains. This observation places some constraints on the ages of the terraces. The terrace deposit in the second trench, for example, was a “high-level” terrace at an elevation of 15.2 m (50 ft) ACL. This terrace was intersected at a depth of 2.6 m (8.5 ft) in auger hole located 3.5 m (11 ft) west-northwest of the second trench. As stated previously, these high-level terraces throughout the Patrick quadrangle are over-lain by sand of the Pinehurst Formation, and thus the terraces are older than the Pinehurst Formation. In this particular trench, the B

b horizon (loamy sand) in the Pinehurst Formation yielded

an OSL age of 29.2 ± 4.9 ka, which is similar to the OSL age of 33.8 ± 3.4 ka obtained from the C horizon (sand) in the Pinehurst Formation of the fi rst trench. It is thought that the top of the B horizon in the second trench (Fig. 24) is the paleosol depicted in Figure 25.End of fi eld trip.Begin drive back to Columbia.

ACKNOWLEDGMENTS

The authors extend sincere thanks to Bill Clendenin, Scott How-ard, and Will Doar of the South Carolina Geological Survey for much cooperation, support, and encouragement. We also

TABLE 1. SOIL SERIES CHARACTERISTICS AND INTERPRETATION OF IMPORTANT UPLAND SANDY SOILS SERIES IN THE MIDDENDORF AND PATRICK QUADRANGLES, CHESTERFIELD COUNTY, SOUTH CAROLINA

rodnaC *dnas niplA :seireS lioS sand* Troupe sand Ailey sand Vaucluse sand Sand (Q) >2 m thick Sand (Q) 1–2 m thick

Sand (Q) 0–1 m thick

?

Buried brown (Q) paleosol

Buried, eroded red (K) weathered sediment Soil classification

Typic quartzi-psamment

Arenic paleudult Grossarenic kandiudult

Arenic kanhapludult

Typic kanhapludult

Data from Morton (1995). *Denotes the soil series used to delineate the boundaries of the Quaternary Pinehurst Formation.



12.1 ± 2.1 ka

sandy loamwith mottling

29.2 ± 4.9 ka loamy sand(buried B horizon)

sand (C horizon)

sand (A horizon)

Figure 24. GPR transect and interpretation of stratigraphy at Stop 10. Location of transect is shown in Figure 3. Paleosol within Pinehurst Formation is the top of the B horizon in the second trench (Fig. 25), and denotes at least one episode of landscape stabilization during accumulation of the Pinehurst Formation.

Figure 25. “Trench 2” at Stop 10, showing pedogenic horizons within the Quaternary Pinehurst Formation. Blue and red tape on hoe mark increments of 0.1 m (0.3 ft).

26 Swezey et al.


thank Allyne Askins and Nancy Jordan of the Carolina Sand-hills National Wildlife Refuge and Brian Davis of the Sand Hills State Forest for property access, logistical help, and additional encouragement. We are grateful for additional property access granted to us by Tommy Wilkes, Kemp McLeod, and Fra-ser Almond. Work by Brad Fitzwater was funded by a USGS EdMap grant, and he thanks Rick Goshen, Steve Stone, Bryte Shoup, Katy Ames, and Ben Hiza for assistance in the fi eld. The Patrick core was obtained by USGS drillers Gene Cobbs, Jeff Gray, and Tony Burkart, and Joe Gellici ran geophysical logs in the core hole. OSL dating was conducted by Shannon Mahan of the USGS and by George Brook of the University of Georgia. Kerby Dobbs of Maser Consulting obtained the GPR data. Frank Dulong of the USGS provided assistance in the identifi cation of clay minerals. This manuscript benefi tted from reviews by USGS geologists Mark Carter and Lucy Edwards. Any use of trade, fi rm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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