Surficial Geology of the Nebraska Portion of the
Naper Quadrangle
R. M. Joeckel, D. Johnson, S. T. Tucker, L. M. Howard, and M. S. Kuzila
Overview of the Nebraska Portion of the Naper Quadrangle
Geographic Setting and Geomorphology
The Naper Quadrangle lies almost entirely within northern Boyd County, Nebraska (a very
small part of it lies within Gregory County in southern South Dakota). Only that portion of
the quadrangle lying in Nebraska is presented as a geologic map in this document.
The Naper Quadrangle lies southward and westward of the Laurentide glacial limit, on the
eastern Great Plains. The northern half and southern one-quarter of the Naper Quadrangle lie
within the United States Department of Agriculture Natural Resources Conservation Service
(NRCS) Major Land Resource Area (MLRA) 66 Dakota-Nebraska Eroded Tableland, whereas
the slopes along the valleys of the Keya Paha and Niobrara rivers are included in MLRA 63B
Southern Rolling Pierre Shale Plains (United States Department of Agriculture, Natural
Resources Conservation Service, 2006).
The topography of the Naper Quadrangle is varied and the landscape is scenic. The total relief
on the land surface within Nebraska portion of the Marty Quadrangle is greater than 450 ft
(137 m), from the lowest elevation in the Niobrara River Valley to the flat summits of hills at
the western margin of the map in section 31 of T. 35 N., R. 15 W. and section 6 of T. 34 N., R.
15 W. Topographic subdivisions within the Naper Quadrangle are as follows:
1. The valleys of the Keya Paha and Niobrara rivers (Fig. 1) in the southern half of the
map, as well as that of Ponca Creek in the northwestern quarter of the map. The
northern valley wall of the Keya Paha River, which is eroded into the Pierre Shale, is
steep and heavily dissected; it exhibits more than 300 ft (91 m) of relief. Pierre Shale is
also ezposed on the lower slopes of the valley walls of Ponca Creek.
2. The dissected flats atop the divide between Ponca Creek and the Keya Paha River, in the
middle of the map area. These flats slope gently southeastward and they are underlain
by Late Pleistocene loess and the Miocene sediments of the Ogallala Formation, and
they have several, small, ovoid deflation basins that contain water during wet periods.
A few, northwest-to-southeast-oriented erosional remnants of Ogallala Group strata rise
from these flats, the highest and most prominent being the Twin Buttes (Fig. 2) in the
southern half of section 12 of T. 34 N., R. 15 W.
3. The dissected uplands eroded from Pliocene sands and gravels on the divide between
Ponca Creek and the Keya Paha River in sections 19, 20, 29, 30, 31, and 32 of T. 35 N.,
R. 15 W. and in sections 5 and 6 of T. 34 N., R. 15 W. This subdivision consists of
gentle to steep hills with prominently flat summits.
4. The eroded ancient fluvial terrace between the Keya Paha and Niobrara rivers in the
southern half of the map. This feature is covered with sands, of both alluvial and eolian
origins, and it is underlain by the Pierre Shale.
Bedrock and Regolith Geology
The reader is referred to Joeckel et. al. (2013) for a discussion of this subject on a regional
basis.
Groundwater Resources
Potential and realized sources of groundwater in the Naper Quadrangle are the very deep,
confined bedrock aquifers of: (1) sandstones in the mid-Cretaceous Dakota Formation, (2) the
Late Cretaceous Codell Sandstone Member (one or more sandstones) in upper part of the
Carlile Shale, (3) the Ogallala Group, and (4) alluvial aquifers in the valleys of Ponca Creek
and the Keya Paha and Niobrara rivers. Sand and gravel aquifers in the Ogallala Group in this
area are strongly limited in thickness and yield, and in contrast to those in Cretaceous strata,
are also very shallow and unconfined. Likewise, the alluvial aquifers are shallow and
unconfined.
Mapping units
Knsh Smoky Hill Chalk Member of the Niobrara Formation (Upper Cretaceous,
Campanian)
Light gray to dark gray shaly chalk, weathering prominently white or brownish yellow; also,
multiple, thin bentonites.
The Niobrara Formation underlies the Pierre Shale in the enclosing region. It crops out only in
the extreme northeastern corner of the Gross Quadrangle, near the Nebraska-South Dakota
border, on lowermost valley slopes along the Missouri River. The Smoky Hill Chalk shows
weak conchoidal fracture, splits into slabs or splinters (rather than fissile chips like the lower
part of the Pierre Shale) and it can be further distinguished by its tendency to weather brownish
yellow or even slightly reddish at the land surface. Fossil oysters (Ostrea) and fish scales can
be found in the Smoky Hill Chalk. Crystals of secondary selenite gypsum commonly appear
on bedding and fracture planes in weathered strata of the Smoky Hill Chalk.
Kp
Pierre Shale (Upper Cretaceous, Campanian-Maastrichtian)
Gray to dark gray (weathering light gray, olive gray, olive brown, grayish brown, pale brown,
and pale yellow) silt shale to clay shale, with rare, very thin to thin beds of siltstone, very fine
sandstone, and bentonite, as well as some beds of chalky limestone.
The Pierre Shale is mapped on the slopes of the river valleys. Approximately 200 ft (61 m) of
upper Pierre Shale strata underlie the north valley wall of the Keya Paha River. These steep
slopes are prone to mass wasting.
Nb
Broadwater Formation (Neogene; Pliocene)
White and light brown, medium to very coarse sands and pebbly to cobbly sands, as well as
gravels.
This unit is mapped in the uplands near the western edge of the quadrangle, where it overlies
strata of the Ogallala Group. Diffendal et al. (2008) mapped the Broadwater Formation at a
very coarse scale of resolution in this part of Boyd County.
Nov
Valentine Formation of the Ogallala Group (Neogene, Middle to Late Miocene)
Dominantly unconsolidated, well-sorted, white, light brown, and pale yellow, fine to medium
sands with a distinct “salt and pepper” aspect due to the presence of dark mineral grains, in
some places containing a few granules and pebbles; also contains thin layers of greenish gray
(weathering to white), indurated opaline sandstones and white, friable, carbonate-cemented
sandstones.
Most of the detailed stratigraphic work identifying the Valentine Formation and differentiating
it from the overlying Ash Hollow Formation was carried out farther west in the Niobrara
Valley (Skinner and Johnson, 1984), and little, if any, attempt has yet been made to
differentiate formations within the Ogallala Group in north-central Nebraska. Herein,
however, we, assign the unconsolidated sands of the Ogallala Group, which are widespread on
the dissected flats atop the divide between Ponca Creek and the Keya Paha River, to the
Valentine Formation of the Ogallala Group. Vertebrate paleontologists have recovered
diagnostic vertebrate fossils in the area of Boyd County for more than 150 years. In 1857, F.V.
Hayden, geologist on the Warren Military Expedition, collected specimens in this area which
were described by Joseph Leidy, but many of these bones lacked adequate geographic or
stratigraphic data. The University of Nebraska State Museum (UNSM) worked a site near the
Twin Buttes (“Forked Hills”; Fig. 2), and replicated many of Hayden’s taxa suggesting this
spot was the source of the material he described in the 1850s. In total, there are nine UNSM
fossil vertebrate localities from Boyd County. All of these sites contain Miocene-age taxa
characteristic of the Barstovian (Ba2) North American Land Mammal Age including
Untermannerix, Megasminthus, Mylagaulus, Eucastor tortus, Aelurodon ferox, Cynarctus
saxatilis, Pliohippus mirabilis, Ustatochoerus medius, Longirostromeryx, and Procranioceras
(Tedford et al., 2004). In the least, this age assignment strongly suggest that strata of the
Ogallala Group in the area are not equivalent to the Ash Hollow Formation.
Large masses and blocks of greenish gray opaline sandstones, which are considered diagnostic
of the Ogallala Group (e.g., Skinner and Taylor, 1967), can be found within or atop these sands
in multiple areas, but particularly on flat hill summits (e.g., along the line between the NE¼ of
section 36, T. 35 N., R. 15 W. and the NW¼ of section 31, T. 35 N., R. 14 W.; and on the
crescent-shaped summit near the shared corner of sections 27, 28, 33, and 34, T. 35 N., R. 15
W.). Nonetheless, there is no landform on the quadrangle that is distinctly capped by a single,
thick, and continuous layer of opaline sandstone.
The Twin Buttes (S½ of section 12, T. 34 N., R. 15 W.; Fig. 2) are capped with a white,
friable, carbonate-cemented, medium sandstone. This sandstone exhibits trough cross strata 25
to 40 cm in thickness that record northeasterly paleoflow, and it also contains common
rhizoliths and invertebrate burrows.
Sediments at the surface of this mapping unit may have been reworked by strong winds during
the Pleistocene, but we interpret the upper surface of the Ogallala Group in the mapped area as
being primarily an erosional surface.
Nofr Fort Randall Formation of the Ogallala Group (Neogene, Middle Miocene)
Massive, laminated, and rippled light gray, yellow, pale yellow, and light yellowish brown
weakly-consolidated siltstones, which locally contain nodules of authigenic carbonate; also
includes beds of light yellowish brown and pale brown well-sorted very fine to fine sands and
lenses of pebble to cobble gravels.
This unit is lithologically distinct from the upper part of the Ogallala Group in the area, and we
have mapped it as the Fort Randall Formation on the basis of a comparison with Skinner and
Taylor (1967), who described the type section of that unit in the Bijou Hills in northeastern
Charles Mix County, South Dakota. Our mapping of the Fort Randall Formation can be
considered provisional pending a complete re-examination of Neogene stratigraphy in the
enclosing region of Nebraska. Fossils from the Fort Randall Formation in its type area place it
within the span of the Barstovian North American Land Mammal Age (16.3- 13.6 Ma).
Sands and gravels are present near or at the base of this unit, and these sediments may produce
small quantities of groundwater.
Qakp1 Very young Quaternary alluvium of the Keya Paha River (post- ca. 1950)
Very pale brown to pale brown, well-sorted very fine sand.
These deposits result from the migration of meander bends in the Keya Paha River since ca.
1950. Individual bends have migrated laterally and downstream some 50 to 100 at some points
during the past six decades, although other stretches have migrated very little, if at all. The
sediments being transported by the modern Keya Paha River are distinctly finer than those
being carried by Ponca Creek, a smaller stream.
Qakp2 Young Quaternary alluvium of the Keya Paha River (Late Holocene)
Very pale brown, pale brown, brownish gray, and grayish brown, well-sorted very fine sand
and silty very fine sand.
These deposits are associated in most places with visible meander scrolls and flood channels.
Most areas mapped at Qakp2 were wooded in the late 1930s and remain at least partially
wooded today, indicating that these areas have been stable for several decades at minimum.
Areas mapped as Qakp2 were indeed present at the time of the first aerial photography of the
region in 1938, but General Land Office (GLO) maps from the 1890s do not show the course
of the Keya Paha in sufficient detail to allow an early historic qualification of the age of these
deposits. These deposits may be as young as early historic, but additional age control is
available. Areas mapped as Qakp2 consist of silty surface strata underlain by sands and they
may be flooded at high flow stages of the Keya Paha River. Mapping units Qakp2 and Qakp3
(below) include a few, very low terraces.
Qakp3 Quaternary alluvium of the Keya Paha River (Holocene)
Very pale brown, pale brown, brownish gray, and grayish brown, well-sorted very fine sand
and silty very fine sand.
Areas mapped as Qakp3 are distal floodplain deposits that appear smooth-surfaced in aerial
photography dating back to 1938. Alluvial deposits in these areas must be older than those in
the more proximal mapping units Qakp1 and Qakp2. Areas mapped as Qakp3 consist of silty
surface strata underlain by sands and it is likely that they are very rarely, if ever, flooded.
Mapping units Qakp2 and Qakp3 include a few, very low terraces.
Qal
Quaternary alluvium of smaller streams (Holocene to modern)
Clayey silt, silt, sandy silt, and sand.
Small, low-order streams in the mapped area deposit clayey silt, silt, sandy silt, and fine to
medium sand. Streams draining basins dominated by surficial materials of either Peoria Loess
or Pierre Shale should be expected to deposit more silt and clay than do other streams. Streams
draining the Pierre Shale may include deposits dominated by coarse sand- to pebble-sized
clasts of shale.
Qan1 Modern alluvium of the Niobrara River
Pale brown and light brownish gray silty sand and fine to coarse sand.
This mapping unit includes recently abandoned channels and accreted bars of the Miobrara
River.
Qan2 Young Quaternary alluvium of the Niobrara River (historic)
Pale brown and light brownish gray silty sand and fine to coarse sand.
Mapping unit Qan2 includes areas along the Niobrara River where bars have been stabilized by
vegetation and accreted since initial Euramerican settlement. These areas may be subject to
flooding during high-stage flow periods of the Niobrara River. Mapping unit Qan3 is likely to
be underlain by Holocene alluvium.
Qan3 Older Quaternary alluvium of the Niobrara River (Late Holocene)
Pale brown and light brownish gray silty sand and fine to coarse sand.
Areas mapped as Qan3 are parts of the floodplain of the Niobrara River, but they are likely to
have been stable since the mid-19th century, if not earlier. These areas may still be flooded
from time to time and may have received comparatively small increments of overbank
sediment since the first Euramerican settlement of the area. Mapping unit Qan3 is likely to be
underlain by older Holocene and Late Pleistocene alluvium.
Qapc1 Youngest Quaternary alluvium of Ponca Creek (modern)
Very pale brown and light brownish gray medium to very coarse sand with pebble gravels
containing common clasts of Pierre Shale ironstone or carbonate nodules and of Ogallala
Group sandstone.
This mapping unit differentiates the modern, entrenched channel belt and bars of Ponca Creek,
which is a small, low sinuosity (S < 1.2) stream in the map area. Ponca Creek is entrenched
into its own alluvium (Qapc2 and Qapc4) within the mapped area. Mature bur oaks (Quercus
macrocarpa) and other trees along the banks of the entrenched stream indicate that it has been
in that state for several decades at minimum. The current channel of Ponca Creek in this area
is, along much of its length, a single-thread planform with alternate bars. Gentle bends in this
planform have wavelengths of 100-150 m. There are, however, slightly braided stretches and a
few, larger meanders with point bars. Immediately downstream from the mapped area, Ponca
Creek is significantly more sinuous and it currently exhibits much larger meanders. The
sediments of mapping unit Qapc1 appear to be inset into those of a larger, predecessor Ponca
Creek, which had large meanders with wavelengths of approximately 1.2 km.
Young Quaternary alluvium of Ponca Creek (Holocene)
Gray, light brownish gray, very pale brown, and brown sandy silt, silty fine sand and fine to
coarse sand.
The areas mapped as Qapc2 are partially wooded and show weak evidence for lateral accretion
in relict point bars and they also exhibit a few flood channels. We speculate that these deposits
represent a higher-discharge regime of Ponca Creek in the past. Since the entrenchment of
Ponca Creek, these deposits are now part of one or more alluvial terraces.
Qapc4 Older Quaternary alluvium of Ponca Creek (Holocene)
Gray, light brownish gray, very pale brown, and brown sandy silt, silty fine sand and fine to
coarse sand.
Areas mapped as Qapc4 are comparatively smooth-surfaced and have been in long-term
agricultural use (likely more than a century). These areas probably never experience overbank
flooding. Since the entrenchment of Ponca Creek, these deposits are now part of one or more
alluvial terraces.
Qas
Alluvial sands on high terraces (Pleistocene)
Very pale brown, well- to moderately sorted, very fine to fine sands containing granules and
pebbles.
In the Naper Quadrangle, deposits mapped as Qas contain few to common granules to pebbles
of crystalline rock types (e.g., granitic rocks and vein quartz), as well as clasts of opaline and
carbonate-cemented sandstones eroded from the Ogallala Group. Multiple clasts of Ogallala
Group sandstone, with what appear to be spherulitic carbonate cements, were found in a
roadcut between sections 20 and 21 of T. 34 N., R. 15 W., at the edge of the ancient alluvial
terrace between the Keya Paha and Niobrara rivers.
Qes1 Eolian sands locally reworked from alluvial terrace sands (Quaternary)
White to very pale brown fine sand.
This unit is mapped in the southern third of the quadrangle atop the ancient fluvial terrace
between the Keya Paha and Niobrara rivers. It is interpreted as wind-reworked alluvial terrace
sediment.
Qes2 Eolian sands locally reworked from alluvial terrace sands, with a surficial silty
component (Quaternary)
White to very pale brown fine sand and silty fine sand.
This unit is mapped in the southern third of the quadrangle atop the ancient fluvial terrace
between the Keya Paha and Niobrara rivers. It is interpreted as wind-reworked alluvial terrace
sediment, but it is also mixed with a small increment of silt that may be of either eolian or
alluvial origin.
Qf
Quaternary alluvial fan sediments (Holocene-modern)
Clay, silt, sand, and gravel, composition depending on source materials.
A few, small, low-relief alluvial fans appear on valley-side slopes and, in one case, on the
dissected uplands at the western margin of the map (around the shared corners of sections 29,
30, 31, and 32 of T. 35 N., R. 15 W.).
Qp1 Peoria Loess (Late Pleistocene [Late Wisconsinan]), typically atop and
amalgamated with shallow sandy sediments of the Ogallala Group.
Light brownish gray, brown, pale brown, and light yellowish brown silt and sandy silt.
This mapping unit represents a thin increment of loess is mapped, as a general rule, over
coarser and more permeable sediments of the Ogallala Group on the uplands; this increment of
loess may be partially amalgamated with the uppermost unconsolidated sediments of the
Ogallala Group, making it difficult to discern as a separate stratigraphic unit.
Qp2 Peoria Loess (Late Pleistocene [Late Wisconsinan]) atop and amalgamated with
shallow sands and gravels of the Broadwater Formation.
Light brownish gray, brown, pale brown, and light yellowish brown silt and sandy silt.
This mapping unit represents a thin increment of loess is mapped over much coarser and more
permeable sands and gravels of the Broadwater Formation.
F
Fill
Artificial fill derived from local sources.
This mapping unit is intended to portray significant accumulations of artificial fill in dams,
road embankments, and other manmade structures.
W
Water
References
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