Surficial Geology of the DuBois 7.5 Minute Quadrangle,
Nebraska
J. T. Korus and L .M. Howard
Mapping units
Qc
Channel deposits (latest Holocene to modern)
clay, silt, sand, and gravel
Unit Qc post-dates the earliest channel dredging and straightening efforts in the Big Nemaha basin,
which occurred primarily from 1904 – 1930 (Rus et al., 2003). These engineering efforts resulted in
measured downcutting of as much as 8 m (26 ft) in parts of the Big Nemaha River channel (Rus et al.,
2003). Qc formed during subsequent lateral migration and widening of the channel. The contact
between Qc and adjacent units is a steep to vertical bank with 8 – 10 m (26 – 33 ft) of relief. It is highly
unstable and is subject to bank failure, especially during large floods. Channel deposits are typically 2 –
3 m (7 – 10 ft) thick near the river edge and thicken distally to 4 – 6 m (13 – 20 ft). They exhibit scroll
topography and are unvegetated near the river edge, becoming partly vegetated with grass, brush, and
trees on older scrolls at slightly higher elevations. Proximal areas of Qc are frequently flooded.
Qal
Alluvium of tributaries of the Big Nemaha River (Holocene to modern)
silty clay, silt, and very fine to fine sand, with basal gravels and coarse sands derived from local bedrock
or till
Unit Qal includes the alluvium of named and unnamed tributaries of the Big Nemaha River. These
deposits vary in thickness from a feather edge to a maximum observed thickness of 12 m (40 ft).
Qan
Alluvium of the South Fork Big Nemaha River (Holocene)
silty clay, silt, and very fine to fine sand, with basal gravels and coarse sands derived from local bedrock
or till
Unit Qan includes alluvium of the South Fork Big Nemaha River. This unit may include some low-relief
features such as terraces and small alluvial fans, but data are insufficient to map these units. Unit Qan is
typically 9 – 12 m (30 – 40 ft) thick. The contact between unit Qan and Qc is a steep bank 4 – 10 m (13 –
33 ft) high and is subject to failure, especially during floods. Gravel and coarse sand typically form the
lowermost 1.5 – 3 m (5 – 10 ft) of this unit.
Qsw
Slope wash and colluvium (late Holocene)
clayey silt, sandy silt, sand, and gravel
Unit Qsw includes slope materials derived from either 1) unconsolidated loess, till, or sand (slope wash),
2) bedrock (colluvium), or 3) a combination of both unconsolidated materials and bedrock. Qsw exists
as thin mantles of sediment along valley margins and in the headwater portions of ephemeral drainages.
Qp
Peoria Loess (late Pleistocene)
slightly clayey silt
Unit Qp is massive to faintly laminated silt and slightly clayey silt. It is brown to yellowish brown, except
for where surface soil is developed, where it is black, dark gray, or dark grayish brown. Qp is present
only on the tops of ridges and on some gently sloping, southeast-facing surfaces in the southeastern
part of the mapped area. Qp ranges in thickness from a feather edge to 2.1 m (7 ft) in test hole 5-B-67.
Small, isolated areas of Qp may be less than 0.3 m (1 ft) thick and present only within the upper horizon
of the modern soil. Qp primarily overlies till, but it may also be present locally atop bedrock units in the
southeastern part of the mapped area.
Qt
Till, slope wash derived from till, and stratified units of till, silty clay, silt, and sand
(Pleistocene [pre-Illinoian])
clay to boulder diamicton
Unit Qt is a matrix-supported, calcareous diamicton containing pebbles, cobbles, and boulders of
limestone, pink (Sioux) quartzite, and igneous and metamorphic rocks. It is dark gray to gray at depth.
The upper part is weathered to yellowish brown. Unit Qt may contain interstratified silts and sands in
some locations. Qt ranges in thickness from a feather edge to more than 12 m (40 ft). It is thickest in
the uplands in the western and northern parts of the mapped area, and the maximum observed
thickness is 32.6 m (107 ft) in test hole 5-B-67.
Pz
Zeandale Formation (Pennsylvanian)
limestone and shale
Unit *z consists of, from base to top, the Tarkio Limestone, Wamego Shale, and Maple Hill Limestone
Members. All three members were encountered in test hole 5-A-67, where it is 8.3 m (27 ft) thick.
Outcropping areas of *z are predominantly the resistive Tarkio Limestone, which forms discontinuous
benches on hillslopes, and the erodible Wamego Shale.
Pw
Willard Formation (Pennsylvanian)
non-calcareous to slightly calcareous shale
Unit *w consists of soft, erodible shale that forms gentle slopes. This unit is present at the surface
primarily in the southwest and northeast parts of the mapped area, although small areas of outcrop
exist in the southeast as well. In the subsurface, *w ranges from 8.5 m (28 ft) in test hole 4-B-67 to 10.4
m (34 ft) in test hole 20-78.
Pbe
Burlingame Formation, Soldier Creek Shale, Wakarusa Limestone, Auburn Formation, and
Emporia Formation (Pennsylvanian)
interbedded limestone, calcareous to non-calcareous shale, and claystone
Unit *be comprises several stratigraphic units of varying lithologies. Only test hole 5-B-67 penetrates
the entire thickness of *be, where it is 13.3 m (44 ft) thick. Resistant limestone units form several
distinct benches on hillslopes. Areas mapped as *be are generally rugged lands covered by forest or
grass.
Ps
Scranton Formation (Pennsylvanian)
noncalcareous shale with minor siltstone, very fine- to fine-grained sandstone, and thin limestone
Unit *s is predominantly very dark gray to dark gray, noncalcareous shale and is 44.5 m (146 ft) thick in
test hole 2-DB-15. Minor amounts of laminated siltstone and heterolithic shale/sandstone are present
locally. The lowermost part is exposed in a stream bank between locations 11227A and 11227B, where
it contains thinly bedded, very fine to fine sandstone and mudstone with abundant, finely disseminated
organic matter. At least two thin to medium beds of limestone and shaly limestone are present in the
upper part of *s. A thin [0.6 m (2 ft)] discontinuous bed of coal is present in the upper part near
location 11217D1.
Ptsh
Topeka Formation, Severy Shale, Howard Limestone (Pennsylvanian)
limestone, shale, siltstone, very fine-grained sandstone, and coal
Unit *tsh comprises three units of varying lithologies. The lowermost unit is the Topeka Formation. It
consist of interbedded limestone, siltstone, and shale, and it includes the Holt Shale, a black, phosphatic
shale that has been correlated throughout the region (Heckel, 1986). The Severy Shale consists
predominantly of shale and heterolithic shale/sandstone. A thin [0.6 m (2 ft)] bed of coal is present near
the top of the Severy Shale at locations 11227A and 11235C and in test hole 1-DB-15. The Howard
Limestone consists of fossiliferous limestone to shaly limestone. The total thickness of *tsh is 17.7 m
(58 ft) in test hole 1-DB-15.
F
Artificial fill
dams, lagoons, or other engineered ground
P
Pit, mine, or quarry (active or abandoned)
excavations, rubble, or disturbed ground related to mining operations
W
Water