Kpy – Yellow Mounds Paleosol equivalent: Terry (1993, 1995a), Terry and LaGarry (1994),
and Terry and others (1995) correlated this unit into Nebraska from southwestern South Dakota,
It consists of pedogenically altered Pierre Shale, and varies in color from yellow to ochre,
orange, pink, and white. The original fissility of the parent shale is almost entirely overprinted
by pedogenic features, which include root traces, ped structures, cutans, mottling, pedogenic
silcretes, calcareous nodules, and redoxymorphic iron concentrations and nodules. APedogenic
limonite frequently occurs 1-2 m below the weathered upper surface. Diagenetic gypsum is also
present 1-2 m below the weathered upper surface where overlain by depositional gypsum in the
Peanut Peak Member of the Chadron Formation [Wolf Butte (Nebraska) 7.5’ quadrangle:
LaGarry and LaGarry (1997n)]. This unit is 1-15 m thick across the northern part of the study
area. Like its parent material, it typically weathers as low hummocks or as steep escarpments in
cutbanks of streams. Outcrops of this unit are frequently covered by cobble lag (stippled on
maps) remnant of the overlying Chamberlain Pass Formation. Its contact with the overlying
Chamberlain Pass Formation is a regional unconformity. Its contact with the underlying
unaltered Pierre Shale is gradational. Although this unit is stratigraphically equivalent to
Retallacks (1983) Yellow Mounds Paleosol Series, which was first described in the Big Badlands
of South Dakota, it likely represents a different soil series. This unit corresponds to the lower
part of Schultz and Stout’s (1955) “Interior Paleosol Complex”.
Qa1 Youngest alluvium (Holocene)- Commonly sand and pebbly gravel, minor thin sandy silt
beds: yellowish-gray and yellowish-orange; unit underlies the modern stream channels and
floodplain. Sands are trough and planar bedded. Most clasts are derived from local bedrock.
Commonly 1-4 m thick.
Qa2 Older alluvium (upper Pleistocene)- Pebbly gravel and sand and local beds of silt.
Deposit underlies a terrace in the southwest corner of the quadrangle about 25-30 m above the
North Platte River.
Qa3 Older alluvium 1 (Pleistocene)
Clays to cobble sized sediment deposited in modern
and ancient stream channels and floodplains.
Qac1 Undifferentiated silty alluvium and colluvium (Pleistocene)
Silt and clay derived
from the weathering of clayey siltstones and silty claystones of the white River Group.
Qac2 Sandy alluvium and colluvium (Holocene and upper Pleistocene)- Sand and silt
residuum and colluvium derived from the weathering of sandy siltstones and sandstones,
primarily of the Arikaree and Ogallala groups; brown to yellowish-gray. Upland occurrences are
primarily colluvium (locally derived gravels or conglomerates) and typically grades into
alluvium. The unit includes the Tassel-Ashollow-Rock soil association. Commonly 2–15 m thick
Qe1 - Unknown Quaternary Unit
(Qe2) Eolian sand- This unit consists of very fine to medium sand deposited by the wind into
sand dunes or sand sheets. The sand dunes are typically transverse dunes with southeast facing
slip faces and are up to 70 m thick. The sand sheets have no recognizable dune forms and are
between 1 and 5 m thick.
Qr1 – Clayey Residuum: This unit consists of residuum and soils derived from the weathering
of the Pierre Shale and minor amounts (10%-15% of alluvial and colluvial clay sediments. This
unit is mapped based on exposures in cut banks and the distribution of soils of the Pierre-KyleSamsil association (Ragon and others, 1977). Residuum or soil < 5 cm thick is mapped as Pierre
Shale. This unit typically has a gradational contact with adjacent Qr2 and Qr3.
Qr2 Silty residuum (Pleistocene)
Residuum and soils derived from the weathering of
silty claystones, clayey siltstones and siltstones of the White River Group and minor amounts
(10-15%) of alluvial and colluvial silty clay sediments.
Qr3 Sandy residuum (Pleistocene)
Residuum and soils derived from the weathering of
sandy siltstones and sandstones of the White River and Arikaree groups and minor amounts (1015%) of alluvial and colluvial silt and sand sediments.
Tam Monroe Creek Formation (Oligocene)
Fine to coarse-grained weakly indurated,
sandstone, volcaniclastic, brown, buff, and gray.
Thin to thickly bedded (0.2-10 m), weakly cross-bedded to massive, and containing locally
abundant siliceous rhizoliths. Carbonate-cemented cylindrical to platy concretions occurring in
zones 0.5 to 2 m thick, are common throughout the unit. Trace fossils in the form of subterranean
social insect colonies are common. Vertebrate fossils are rare, and only fragments of tortoise
carapace were recovered during the mapping. The lower contact where visible, is a sharp
erosional contact with the Sharps Member of the Brule Formation. This unit is between 150-190
m thick depending how the upper contact was established (see above).
Twbh – Horn Member: This provisional lithostratigraphic unit (Swinehart, unpublished data) is
named for the small community of Horn 16 km northwest of Crawford, Nebraska [Horn
(Nebraska) 7.5’ quadrangle]. It consists of massive to weakly stratified buff, pinkish gray, or
pale brown volcaniclastic siltstone, volcaniclastic silty sandstone, and volcaniclastic sandy
siltstone. At its proposed type section at the head of Whitehead Creek near Roundtop [Roundtop
(Nebraska) 7.5’ quadrangle: LaGarry and LaGarry (1997n)], this unit is about 55 m thick, but
thins to 5-10 m thick to the west [Sherrill Hills (Wyoming-Nebraska) 7.5’ quadrangle] this unit is
distinctly thinly bedded. This unit erodes into vertical cliffs or rounded ledges having a highly
fractured appearance and occasionally contains small (2-10 cm), irregularly shaped nodules or
concretions in discontinuous zones 1-10 m thick. A prominent volcanic ash, the Nonpareil Ash
Zone (NPAZ) of Swinehart and others (1985), occurs 5-33 m above the base of the unit. This
ash has been 40Ar/39Ar dated at 30.050 ± 0.19 Ma (Swisher and Prothero, 1990). Vertebrate
fossils are locally abundant at its base, and include oreodonts, rhinoceroses and tortoises. This
unit has a sharp erosional contact with the overlying Arikaree Group. Below the east-west
trending Arikaree Group paleovalley (see above), this unit is incised by the “Ash Creek Beds”.
Elsewhere, this unit is overlain by the Monroe Creek Formation. The lower contact with the
Whitney Member is either gradational or an erosional unconformity where this unit fills valleys
and depressions in the underlying Whitney Member. This unit corresponds to the basal part of
Schultz and Stout’s (1955) and Meehan’s (1994) “Gering Formation” [Roundtop (Nebraska) 7.5’
quadrangle: see LaGarry and LaGarry (1997n)], and the “Roundtop member” of the Brule
Formation of Terry and others (1995).
Twbo – Brule Formation, Orella Member: This unit consists of thinly interbedded brown,
orange, and buff volcaniclastic clayey siltstones, volcaniclastic silty claystones, discontinuous
medium to coarse grained, crossbedded and massive, brown sheet sandstones, and multistoried
basal channel sandstones. The clayey siltstone, silty claystone, and sheet sandstone beds usually
occur as couplets <0.5 m thick, with each couplet consisting of a thin, discontinuous sheet
sandstone and an associated pedogenically altered clayey siltstone or silt claystone. The only
pedogenic features with the silty claystone and clayey siltstones consist of chalcedony-filled root
voids and insect tunnels. Terry and others (1995) described these weakly developed paleosols as
“entisols”. The distribution of the discontinuous sheet sandstones is controlled by a complex
overlapping cut-and-fill geometry (see Wells, 1994; Wells and others 1994, 1995; Terry and
others, 1995; LaGarry and LaGarry, 1997n). The sheet sandstones are thinnest, poorly
developed, or absent in the basal one-third of the paleovalley fills (Wells, 1994; Wells and
others, 1995). Common sedimentary structures within these sheet sands include soft sediment
deformation, fluid escape structures, planar crossbeds, mudcracks or subaqueous shrinkage
cracks, and invertebrate trace fossils (Terry and others, 1995; Wells and others, 1995). These
“nodular” appearing sheet sands were interpreted as pedogenic nodules by earlier workers
(Schultz and Stout, 1955; Schultz and others, 1955). A 0.01 m thick volcanic ash bed (the
“Serendipity Ash”) occurs 16 m above the base of the unit at the level of Schultz and Stout’s
(1955) “Diplolophus insolens bench”. The basal multistoried channel sandstone (the “Toadstol
Park Channel complex” of Schultz and Stout, 1955) is 0-9 m thick, and consists of overlapping
paleochannels with trough and planar crossbedded, medium to coarse grained, clay matrixsupported sandstones having a gravel lag, thinly bedded and laminated green sandstones, and
trough crossbedded conglomeratic sandstones having a gravel lag that contains small fragments
of vertebrate bone and turtle carapace. These basal sandstones contain a suite of sedimentary
structures consistent with a “mixed” braided/meandering river system, including trough
crossbeds and lateral accretion surfaces (Terry and others, 1995). They are exposed
intermittently within the Roundtop (Nebraska), Horn (Nebraska), Bodarc (Nebraska), and
Warbonnet Buttes (Nebraska) 7.5’ quadrangles (see also LaGarry and LaGarry, 1997a).
The uppermost surface of the “Toadstool Park Channel complex” contains abundant
vertebrate and invertebrate trace fossils, including tracks and trails of worms, insects,
crustaceans, birds, rhinoceroses, entelodonts, camels, rodents, lagomorphs, oreodonts, and
carnivores (Nixon and LaGarry-Guyon, 1993a, 1993b; Terry and others, 1995). Vertebrate
fossils are common within the silty claystones and clayey siltstones. The overall thickness of
this unit is 26-29 m, and it weathers into steep, step-and-riser badlands. This unit thins to the
east [Chadron West (Nebraska) 7.5’quadrangle] and west [Sherrill Hills (Wyoming-Nebraska)
7.5’ quadrangle] to 2-4 m of thinly bedded buff or brown siltstone. This unit thins to the east
[Chadron West (Nebraska) 7.5’quadrangle] and west [Sherrill Hills (Wyoming-Nebraska)
7.5’quadrangle] to 2-4 m of thinly bedded buff or brown siltstone. This unit has an intertonguing
contact with the overlying Whitney member, except where incised by paleochannels within the
lower Whitney Member. It has an intertonguing contact with the underlying “Big Cotton wood
Creek member” of the Chadron Formation, except where the underlying Chadron Formation is
incised by the “Toadstool Park Channel complex.” This unit is equivalent to Schultz and Stout’s
(1955) “Orella B” and “Orella C.”
Twbw Whitney Member (upper Oligocene)- Siltstone; volcaniclastic; grayish orange and
yellowish brown; predominantly interpreted as eolian in orgin. Unit typically very thick bedded
to massive. Siltstone cemented with clay and carbonate. Member is known to have two
regionally correlative ash beds; only the biotitic lower ash (LW ash), up to 1.5 m thick, was
positively identified. Commonly 50-100m thick.
Twc1 – Big Cottonwood Creek Member: This new lithostratigraphic unit (Terry and LaGarry,
1998) is named for prominent exposures along Big Cottonwood Creek near Toadstool Geologic
Park [Roundtop (Nebraska) 7.5’ Quadrangle: LaGarry and LaGarry (1997); see also Terry and
others (1995)]. It consists primarily of pedogenically altered volcaniclastic silty claystones and
minor amounts of interbedded claystone, pedogenic carbonate, lacustrine carbonate, massive
lenticular sandstone, cross and planar bedded sandstone, diagenetic and sedimentary gypsum,
volcanic ash, and a multistoried basal channel sandstone. Weakly developed paleosols in its
uppermost 20 m give this unit a characteristic banded appearance; common colors are yellow,
buff, pink, tan, green and red. The lowermost 15-20 m has less paleosol development, with
common colors being green, yellow and red. In addition to carbonates (typically calcretes),
pedogenic features within this unit include ped structures, cutans, and root traces ( Terry, 1995b).
The pedogenic carbonates are <0.5 m thick and lenticular or tabular. Many contain fossilized
dung rolled into balls by insects prior to deposition. The lacustrine carbonates are thin(<0.3 m
thick), tabular, and discontinuous, and contain algal laminae, ostracodes, plant macrofossils, and
fish remains (see also Welzenbach and Evans, 1992). Lacustrine carbonates with a high silt or
clay content are considered marls (Terry and others, 1995; LaGarry and LaGarry, 1997). The
massive and planar bedded sandstones have a wide geographic distribution (see LaGarry and
Terry, 1997). However, most of the diagenitic gypsum (often occurring as psuedomorphs) and
all of the sedimentary typsum (beds 0.2-0.6 m thick0 are restricted to the Wolf Butte (Nebraska)
7.5’ quadrangle (LaGarry and LaGarry, 1997n). The gypsum, sandstone, limestone, marl, and
claystone beds are most common within the lower 15-20 m of the unit. Multistoried basal
sandstones 2-5 m thick occur locally [Sherrill Hills (Nebraska), Story (Nebraska), and Story NE
(Nebraska), and Roundtop (Nebraska) 7.5’ quadrangles: see also LaGarry and LaGarry (1997)].
At least five prominent volcanic ashes are present throughout this unit. The uppermost ash,
Schultz and Stout’s (1955) “upper purplish-white layer” is locally widespread marker bed. The
next lower ash, Schultz and Stout’s (1955) “lower” or “second purplish-white layer” is less
widespread, and often only slightly more vitric than the adjacent volcaniclastic silty claystones.
The lower three ashes are lenticular and really restricted, occurring within paleovalleys incised
into the underlying Peanut Peak Member. The lowermost 20 m of this unit typically occurs as
isolated erosional remnants within these paleovalleys [Horn (Nebraska) 7.5’ quadrangle]. This
unit is calcareous in zones, giving it a nodular (5-10 cm diameter) appearance. It is also highly
fossiliferous, containing abundant fossil vertebrate and infrequent fossil invertebrate remains.
The overall thickness of this unit is 22-40 m; it is thickest where it has incised into the
underlying Peanut Peak Member. This unit typically weathers into smooth, near vertical faces
and badlands. It has an intertonguing contact with the overlying Brule Formation, except where
it is incised by the “Toadstool Park Channel complex” of the Orella Member of the Brule
Formation. It has an intertonguing contact with the underlying Peanut Peak Member, except
where this unit incises or fills valleys and depressions in the underying Peanut Peak Member.
This unit is equivalent to Schultz and Stout’s (1955) “Chadron B3”, “Chadron B4”, “Chadron C”,
and “Orella A”. This unit does not occur east of the Trunk Butte (Nebraska) 7.5’ quadrangle,
where this stratigraphic position is occupied by the “Trunk Butte beds” of the Chadron
Formation (LaGarry and LaGarry, 1999).
Twc2 – Chadron Formation, Peanut Peak Member: Terry (1995, 1998), Terry and LaGarry
(1998), and Terry and others (1995) correlated this unit into Nebraska from southwestern South
Dakota. It consists of dark olive, gray, yellow and red claystones. Most primary sedimentary
structure within the claystones (e.g., lamination) are overprinted by pedogenic features, which
include root traces, ped structures, cutans, mottling, and calcareous nodules. Discontinuously
bedded gypsum, lacustrine limestone and marl, pedogenic calcrete, and sandstone beds occur in
different locations and at various stratigraphic levels within the unit. Gypsum beds occur as
diagenetically emplaced (0.1-0.5 m thick) and as a result of primary deposition (0.1-0.25 m
thick) [Wolf Butte (Nebraska) 7.5’ quadrangle: see LaGarry and LaGarry (1997)]. Lacustrine
limestones are tabular (0.05-0.25 m thick), are widespread both geographically and
stratigraphically, and contain algal laminae, ostracodes, plant macrofossils, and fish remains (see
also Welzenbach and Evans, 1992). The calcretes are restricted to the Horn (Nebraska), Trunk
Butte (Nebraska), and Chadron West (Nebraska) 7.5’ quadrangle (see also LaGarry and LaGarry,
1997; LaGarry and Terry, 1997). Isolated sandstones are massive, lenticular or tabular,
crossbedded, gypsiferous (0.1-1.5 m thick), and occur west of Toadstool Park [Roundtop
(Nebraska) 7.5’ quadrangle: LaGarry and Terry (1997)]. At least one, and possibly as many as
four lenticularly bedded vocanic ashes (0.1-0.3 m thick) are present throughout the unit. This
unit has a thickness of 2-17 m. Its contact with the overlying “Big Cottonwood Creek member”
is intertonguing except where incised by the basal channel sandstones of the “Big Cottonwood
Creek member” or eroded into paleovalleys. Its lower contact with the underlying Chamberlain
Pass Formation is a regional unconformity. This unit weathers into smooth, low hummocks, and
is equivalent to Schultz and Stout’s (1955) “Chadron B1” and “Chadron B2”, and Clark and
others’ (1967) “Chadron Formation undifferentiated” and Peanut Peak Member of the Chadron
Formation in the Big Badlands of South Dakota.
Twcp – Chamberlain Pass Formation: Terry (1993, 1995a), Terry and LaGarry (1994), and
Terry and others (1995) correlated this unit into Nebraska from southwestern South Dakota. It
consists of pedogenically altered red, gray, white, yellowish olive, and olive silty claystones and
interbedded lenticular single and multistoried white, gray, yellow, yellowish green, or green
trough crossbedded channel sandstones. The claystones [“mudstones” of Evans and Terry
(1994) and Terry and Evans (1994)] are laminated or massive and typically 1-3 m thick. The
claystones contain Terry’s (1993, 1995a) dark red Interior Paleosol Series equivalent (see
Retallack, 1983) and yellowish olive Weta Paleosol Series equivalent (see Evans and Terry,
1994; Terry and Evans, 1994). Most primary sedimentary structures (e.g., lamination) are
overprinted by pedogenic feature, which include root traces, ped structures, cutans, mottling,
pedogenic silcretes, calcareous nodules, and redoxymorphic iron accumulations. Calcareous
nodules (2-5 cm diameter) occur in descrete horizons. The sandstones are single (0.5-1.0m
thick) and multistoried (2-8 m thick), fine to coarse grained, pedogenically altered, and usually
contain a gravel to cobble sized lag at base. Pedogenic alteration of the sandstones has
transformed their feldspars into kaolinitic clays, an based on the amount of kaolinite within these
sandstones, they were likely arkosic when deposited (Terry and Evans, 1994). Individual
sandstone bodies are trough crossbedded, fine upwards, and contain pebble stringers, intraclasts,
and large blocks of reworked red and yellowish olive claystone and shale incorporated into their
base. The overall thickness of this unit is 1-13 m. Its contacts with the overlying Chadron
Formation and the underlying Yellow Mounds Paleosol Series equivalent are regional
inconformities. Along Whitehead Creek in the Five Points (Nebraska), Montrose (Nebraska),
and Roundtop (Nebraska) 7.5’ quadrangles (see also Terry and others, 1995; LaGarry and
LaGarry, 1997n) this unit had incised into underlying unaltered Pierre Shale. The claystones
weather into low, smooth hummocks, and the channel sandstones weather into ledges and
vertical cliffs. This unit corresponds to the basal gravels and “beds of a bright dark red color” of
Darton’s (1899) original Chadron Fromation, the upper part of Schultz and Stout’s (1955)
“Interior Paleosol Complex” (the Interior Paleosol Series equivalent), and Schultz and Stout’s
(1955) “Chadron A” or “Yoder” (the channel sandstones: Terry and others, 1995; LaGarry and
LaGarry, 1997n).