The document discusses lithified sedimentary rocks and their impact on groundwater flow. Complex stratigraphy, such as interfingering rock units, can make groundwater exploration challenging. Folds and faults can create complex hydrogeologic systems, acting as barriers or conduits to flow depending on their composition. Clastic sedimentary rocks have permeability influenced by grain size, sorting, and cementation, with fractures increasing permeability. Fractures near the surface are important for bypassing low-permeability layers and recharge.

1. 8.3 Lithified Sedimentary Rocks 299
Lake
Michigan
Wisconsin
Illinois
Scale:
A FIGURE 8.12
Area where Maquoketa shale is thin or missing and direct
recharge to the CambrianOrdovician aquifer can occur.
Source: C. W. Fetter, Jr., Ground Water 79 (1981): 201—
13. O Ground Water Publishing Company. Used with
permission.
i«etric in theaq#r hasdèîlinèd
in ibis
thè

2. 300 Chapter Geology of Ground-Water Occurrence
8
750
Potentiometric surface of Cambrian-Ordovician aquifer in
southeastern Wisconsin and northeastern Illinois. A. About
1865—1880. B. In 1973. Source: C. W. Fetter, Jr., Ground
Water 7 9 (1981): 207—13. © Ground Water Publishing Company.
Used with permission.
8.3.1 Complex Stratigraphy
The fact that a formation may change in lithology from one locality to another accounts
for some of the difficulties associated with studies of sedimentary rock units. For
example, a sandstone may grade into a shaly sandstone or siltstone, yet still have the
same fossil fauna assemblage and the same stratigraphic nomenclature. The Eau Claire
Formation of Cambrian age is a sandstone in east-central Wisconsin, but it grades into a
shale and siltstone in northern Illinois. In Wisconsin, it has high conductivity and is an
aquifer; in Illinois, it becomes a confining layer (Walton 1965; Schicht, Adams, & Stall
1976).
Complex stratigraphy can be a very real hindrance to ground-water exploration. On
the Hualapai Plateau of northwestern Arizona, the best potential aquifer is the
stratigraphic sequence of sedimentary rocks in the Rampart Cave Member of the Muav
Limestone Formation (Twenter 1962). Springs discharge from this member into the
Grand Canyon along the flanks of the plateau. The Bright Angel Shale stratigraphically
underlies the Rampart Cave Member of the Muav Limestone, causing the ground water
to be perched (Figure 8.14). However, because these units were being deposited in a
800M
850k'
A FIGURE
8.1
3

3. transgressing sea, the various beds are interfingering. As a result, at some localities the
Bright Angel Shale can be both above and below the Rampart Cave Member (Figure
8.15). Drillers in the area have had unsuccessful wells because they stopped drilling
when the Bright Angel
Unit Age Thickness (ft) Lithology
Gravels,
Basalt, Tuff
esco ame
ormat10
Tertia 0—1000
2
Manakacha
Formation
Wata omi i
Formatio
250
200
Redwall
Limestone 600-675
Temple Butte
Formation
n tvi
Devon ian 400-450
1
Muav Limestone 800-1200
Bright Angel
Shale
400—
450
apeats andstone
Vrshnu Schist Precambrian
EXPLANATION
Sandstone- Shale Limestone Dolomite Chert
Siltstone
Gravel Igneous- Basalt-Unconformity
Metamorphic Tuff
A FIGURE 8.14
Stratigraphy of the Grand Canyon area. Source: P. W. Huntoon,
Ground Water 15 (7 977): 426—33. © Ground Water Publishing
Company. Used with permission.

4. 302 Chapter Geology of Ground-Water Occurrence
Bright
Angel
Shale
A FIGURE 8.15
Interfingering of sedimentary rock units of the Hualapai
Plateau area. Source: Adapted from P. W. Huntoon, Ground
Water 15 (7 977): 426—33. © Ground Water Publishing
Company. Used with permission.
301
8
Shale was first reached (Huntoon 1977). Unfortunately, the target formation had not yet
been penetrated. A well drilled at Location A of Figure 8.15 would not hit Bright Angel
Shale until the Rampart Cave Member was penetrated. However, at location B, three
shale members alternating with limestone would have to be penetrated before the
Rampart Cave Member was reached. The area of these interfingering members extends
for tens of miles. The hydrogeologist working in areas of such complex stratigraphy must
be aWare of the possible hydrogeologic consequences.
8.3.2 Folds and Faults
Folding and faulting of sedimentary rocks can create very complex hydrogeologic
tems, in which determination of the locations of recharge and discharge zones and flow
systems is confounded. Not only must the hydrogeologist determine the hydraulic
characteristics of rock •units and measure ground-water levels in wells to determine flow
systems, but detailed geology must also be evaluated. In most cases, the basic geologic
structure will have already been determined; however, logs of test wells and borings must
be reconciled with the preexisting geologic knowledge.
Fault zones can act either as barriers to ground-water flow or as ground-water
conduits, depending upon the nature of the material in the fault zone. If the fault zone
consists of finely ground rock and clay (gouge), the material may have a very low
hydraulic conductivity. Significant differences in ground-water levels can occur across
such faults (see Figures 8.9 and 8.10). Impounding faults can occur in unconsolidated
materials with clay present, as well as in sedimentary rocks where interbedded shales,
which normally would not hinder lateral ground-water flow, can be smeared along the
fault by drag folds. Clastic dikes are intrusions of sediment that are forced into rock
fractures. If they are clay-rich, they can act as ground-water barriers in either sediments or
in a lithified sedimentary rock. Clastic dikes are known to occur in alluvial sediments,
glacial outwash, and lithified sedimentary rock.
Faults in consolidated rock units can act either as pathways for water movement or as
flow barriers. If there has been little displacement along the fault, then the fault is more
likely to develop fracture permeability because there is less opportunity for the formation
of soft, ground-up rock, called gouge, to form between the moving surfaces (Huntoon
1986). Fault gouge can have a matrix of rock breccia encased in clay and can have a wider
range of permeability (Snipes et al. 1986). Faults in poorly consolidated rocks in South
Muav Limestone
—

5. Carolina had greater permeability than those in well-consolidated rocks (Snipes et al.
1986).
If the fault zone has a high porosity and hydraulic conductivity, it can serve as a
conduit for ground-water movement. Springs discharging into the Colorado River in
Marble Canyon are controlled by a vertical fault zone, the Fence Fault. The springs
discharge where the faults intersect the river. The fault zones provide for vertical
movement of recharging ground water from the land surface as well as lateral movement
toward the river. The geochemistry of the spring water indicates that some of the water
discharging on one side of the river originated in the ground-water basin on the opposite
side of the river, indicating the fault zone was conducting some ground-water flow
beneath the river even though it is a regional discharge zone (Huntoon 1981).
Faults may contain ground water at great pressures at depths where tunnels or mines
may be constructed. One danger of hard-rock tunneling is the possibility of breaching an
unexpected fault zone. Damaging and dangerous flooding can occur if the fault contains
ground water with a high hydraulic head. In Utah, a well being drilled through an
anticlinal structure, which in an unfaulted state creates a regional confining layer,
encountered an exceptionally high-permeability zone created by normal faults and
associated extensional joints that imprinted joint permeability on the brittle rocks. The
well was being

6. 304 Chapter 8 Geology of Ground-Water
Occurrence
Tertiary
cover
HOMOCLINAL
MARGIN
5000 ft
15—60 miles
MOUNTAIN
UPLIFT
10,000 ft
-
SEVERED
MARGIN
Tertiary
cover
s ic roc
PalQ0R9iç: rqqks
FIGURE 8.16
Schematic cross section through a typical Wyoming mountain
uplift showing the style of deformation that results in
approximately equal percentages of fault severed and
homoclinal basin perimeters. Source: P. W. Huntoon, Ground
Water 23 (7 985): 7 76—81. C) Ground Water Publishing Company.
Used with permission.
drilled to initiate solution mining of an abandoned underground potash mine.
Unfortunately, ground water under high pressure in the fracture zone rushed into the well
boring when the mine level was reached and flooded it with mineralized water (Huntoon
1986).
Overthrust faulting can create conditions in which a rock, normally found as an
impermeable basement unit, is overlying the sedimentary rock units, typically a
groundwater source. In such a case, the hydrogeologist might recommend drilling through
the "basement" rocks to attempt to obtain a ground-water supply from younger
sedimentary units, provided there is an opportunity for recharge to occur and the water is
not known to have a high mineral content (Figure 8.16).
The major artesian basins of Wyoming consist of sedimentary rock units bounded by
major thrust faults. The fault-severed margins of the basins have good permeability,
adequate recharge, and good-quality water in the sedimentary rock aquifers of the hanging
wall. However, the foot-wall rocks receive little recharge, have poor-quality water, and
have permeabilities often many orders of magnitude lower than those of the adjacent
hanging wall segments of the same formation (Huntoon 1985).
Is tog-round-V@ter flow,
W
p,

7. 8.3 Lithified Sedimentary Rocks305
Ðtþrbekisunižotmlyfragturèdw1 depthÅåtRmoreextenFiÿe fractured-zones inthe rock
LEGEND
O PRODUCTION
MONITORING WELL
FAULT
Eastern
Fault Block
North Main
Cross-Fault
Eastern
P23 Fault Block
T24 P22
Ml
Central Il
Fault Block Bl
Cross-Fault
T33
T31 Central I
P21
Fault Block
TORY
P32 Eastern Fault
Western o
Fault Block rapids
Central Fault
200m
Western Fault scale
A FIGURE 8.17
Locations of wells, faults, and fault blocks, Carlton University.
Source: Allen, Diana M. and Frederick A. Michel. 1998. Ground Water
36. no. 6:938—948 © Ground Water Publishing Company. Used with
permission.
indicate
theßyOf4@lt blocks
ssivities. The centfrål zone zone had a
transmissivitÿ bf aŽ56

8. 306 Chapter 8 Geology of Ground-Water
Occurrence
Séne that 9.26 separates ft /daÿ
(0.86 the mtwo 2
/daÿOaquifer
þIoçksÁÅta' of
Folding can affect the hydrogeology of sedimentary rocks in
several ways. The most obvious is the creation of confined
aquifers at the centers of synclines. The nature of the fold
will affect the availability of water. A tight, deeply plunging
fold might carry the aquifer too deep beneath the surface to be
economically developed. Deeply circulating ground water is also
typically warmed by the geothermal gradient and may be highly
SIMPLIFIED STRATIGRAPHIC PROFILE
ALONG THE LINE
A-AI
60
mast 0
LEGEND
shale
siltstone
sandstone
100limestone
dolostone
calcaren
200
SIMPLIFIED STRATIGRAPHIC PROFILE
ALONG THE LINE B-B I
60
masl O
100

9. 8.3 Lithified Sedimentary Rocks307
200
B
A FIGURE 8.18
Stratigraphic cross sections across Carlton University campus.
Cross section locations shown on Figure 8.16. Source: Allen,
Diana M. and Frederick A. Michel. 7998. Ground Water 36. no.
6:938—948. © Ground Water Publishing Company. Used with
permission.
mineralized. A broad, gentle fold can create a relatively
shallow, confined aquifer that extends over a large area. This
might be a good source of water if sufficient recharge can
occur through the confining layer or if the aquifer can
transmit enough water from areas where the confining layer is
absent.
Another effect of folding is to create a series of outcrops
of soluble rock, such as limestone, alternating with rock units
that are not as permeable. Smaller streams flowing the
limestone might sink at the upper end, only to reappear at the
lower outcrop. The type of trellis drainage that can develop on
folded rocks is shown in Figure 8.20. Surace streams follow the
strike of rock outcrops, usually along fault or fracture
traces. In •olded sedimentary rocks with solutional conduits in
carbonate units, ground-water flow nay be along the conduits
that parallel the strike of the fold and not down the dip.
In areas of homoclinal folds, the outcrop areas usually have
bands of sedimentary ocks, with resistant rocks forming ridges
and more easily erodable rocks forming valleys.

10. 308 Chapter 8 Geology of Ground-Water
Occurrence
A FIGURE 8.19
Static hydraulic head measurements in wells on Carlton
University campus. Data from Allen, Diana M. and Frederick A.
Michel. 1999. Ground Water 37. no, 5: 718—728.
The ridges may create ground-water divides, with aquifers outcropping in the valleys. The
outcrop area of an aquifer will have local water-table flow systems with relatively large
amounts of water circulating. These areas also serve as the recharge zones for the more distal
parts of the aquifer, which are downdip in the basin and confined. There is a limited amount of
natural discharge from the confined portions of the aquifer, this is, typically upward leakage
into overlying beds with lower hydraulic head. Because of poor groundwater circulation, the
confined portions of the aquifer may have low hydraulic conductivity and poor water quality
(LeGrand & Pettyjohn 1981).
Complex folded and faulted sedimentary rock units are a challenge to the hydrogeologist.
Competency in geology is necessary in order to construct geologic cross sections based on
well logs, drill-core samples, and outcrops. Regional flow systems can be controlled by the
structural and stratigraphic relations of the confining beds and aquifer units. In addition,
distribution of hydraulic potential must be determined, very often from limited data.

11. 8.3 Lithified Sedimentary Rocks309
FIGURE 8.20
Drainage pattern developed in an area of
longitudinally folded rock strata: A. Top view. B.
Cross section.
Birds-eye
view
Fracture trace
Cross section
8.33 Clastic Sedimentary Rocks
Hydraulic conductivity of clastic sedimentary rocks, based on
primary permeability, is a function of the grain size, shape,
and sorting of the original sediment. The same factors that
affect the permeability and porosity of loose sediments also
are important in sedimentary rocks. Cementation, in which parts
of the voids are filled with precipitated material such as
silica, calcite, or iron oxide, can reduce the original
porosity. Solution of the original material or cementing agent
may occur during and after diagenesis, resulting in an increase
in porosity.
Consolidated rocks also contain secondary porosity and
permeability due to fracturing. Microfractures may add very
little to the original hydraulic characteristics; however,
major fracture zones may have localized hydraulic
conductivities several orders of magnihide greater than that of
the unfractured rock. Fracturing can occur through several
geoogic processes. Rock at depth is under great pressure owing
to the weight of the )verburden. As uplift and erosion bring
the consolidated rock to the surface, it expands as he pressure
is reduced. The expansion can cause fracturing of the rock,
o o o o
o

12. 310 Chapter 8 Geology of Ground-Water
Occurrence
with the majority the expansion fractures occurring within
about 300 ft (100 m) of the surface. Vertical ractures carry
recharging precipitation downward and provide a very important
function n bypassing low-permeability layers near the surface.
Wells located in surface fracture ones are generally highly
successful.
c
A FIGURE 8.21
Sedimentary conditions producing a sandstone aquifer of variable thickness: A. Sandstone
deposited in a sedimentary basin. B. Sandstone deposited unconformably over an erosional
surface. C. Surface of sandstone dissected by erosion prior to deposition of overlying beds.
Fracturing may also be associated with tectonic activity. Rock deformed by faulting or
folding may fracture when subjected to tension or compression. Such activity can take place at
substantial depths; thus, secondary permeability is not strictly a near-surface phenomenon.
However, great pressure on the deeper fractures does not permit them to be as open (have as
great a porosity) as shallow fractures.
The yield to wells is proportional to the transmissivity of the aquifer. This, in turn, is
proportional to the aquifer thickness if the hydraulic conductivity is uniform throughout the
aquifer. Sedimentary aquifers were deposited in sedimentary basins in which units gradually
thicken. Variable thickness of a sedimentary aquifer may also be due to the deposition of the
aquifer material over an eroded surface with high relief or a dissection of the top of the aquifer
after deposition (Figure 8.21). Higher well yields will be obtained from thicker sections of the
aquifer. The relationship between the specific capacity of wells and the uncased thickness of
two sandstone aquifers in northern Illinois is shown in Figure 8.22.
Wells in sandstone aquifers should be located in such a manner as to penetrate the
maximum saturated thickness of the aquifer. If one area of the aquifer is known to have a
higher hydraulic conductivity than other areas, the combination of hydraulic conductivity and
Sandstone

13. 8.3 Lithified Sedimentary Rocks311
thickness should be considered in order to locate the well in the area of greatest aquifer
transmissivity.
The yield of sandstone wells can sometimes be increased by the detonation of explosives
in the uncased hole. The shots are generally located opposite the most permeable zones of the
sandstone. The loosened rock and sand is bailed from the well prior to the installation of the
pump. The shooting process has two effects on the borehole: it enlarges