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GEOLOGY
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June 2014
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www.gsapubs.org 479
INTRODUCTION
The formation of layer-bound normal faults
and shear fractures arrayed in broadly polygonal
networks is recognized as a common occurrence
during the early burial history and diagenesis of
very fi ne-grained sedimentary sequences. Over
the past 20 years, polygonal fault systems have
been discovered using two- and three-dimen-
sional (2-D and 3-D) seismic data in >100 sedi-
mentary basins worldwide, almost exclusively
in the subsurface in marine settings on conti-
nental margins (Cartwright, 2011).
Despite the fact that polygonal fault systems
are remarkably common in modern and ancient
marine basins, few on-land exposures have been
documented that lend themselves to fi eld study.
Polygonal faults have been proposed to occur in
Paleogene claystones in Belgium (Verschuren,
1992) and in discontinuous, widely separated
outcrops and quarry exposures of Cretaceous
chalk in the UK and France (Hibsch et al., 2003).
Chalk of the Khoman Formation near Far-
afra Oasis, Egypt (Fig. 1), contains thousands
of narrow, low-relief ridges exposed nearly
continuously over a minimum of 900 km2 and
extending, partly mantled by eolian sand, over
at least 1800 km2. Our work with high-resolu-
tion satellite imagery shows that these ridges
are arrayed in a polygonal network with geom-
etries and scales similar to those of polygonal
faults studied using 3-D seismic data. Our fi eld
analysis has documented an extensive tract of
polygonal faults and related features, allow-
ing us to characterize the 3-D architecture of
a polygonal fault system in chalk, small-scale
fault zone features, the formation mechanics,
and the role of fl uids in fault system evolution.
POLYGONAL FAULT SYSTEMS
Polygonal fault systems consist of an array of
layer-bound normal faults that partially or fully
intersect in broadly polygonal patterns. Faults
of different orientations in a polygonal system
have been suggested to form synchronously in
an essentially isotropic horizontal stress fi eld
(Shmax = Shmin) (Cartwright and Dewhurst, 1998;
Cartwright et al., 2003; Hibsch et al., 2003; Gay
et al., 2004; Cartwright, 2011). Although strains
are radially isotropic and extensional, layers con-
taining polygonal faults are suggested to have
undergone no net extension because host rocks
undergo simultaneous volume loss accompanied
by fl uid expulsion. As the layer shrinks radially,
polygonal networks of small-scale and meso-
scale normal faults develop.
Polygonal faults form early in the burial histo-
ry of fi ne-grained sediments. Although the vast
majority previously described occur in clay-rich
sediments, polygonal fault systems have been
reported in chalk (Hibsch et al., 2003; Hansen
et al., 2004). Faults extend nearly to the seafl oor
in some systems, and polygonal fault systems
continue to develop as burial increases, with
compaction-induced rotation resulting in shal-
Polygonal faults in chalk: Insights from extensive exposures of the
Khoman Formation, Western Desert, Egypt
Barbara J. Tewksbury1, John P. Hogan2, Simon A. Kattenhorn3, Charlotte J. Mehrtens4, and Elhamy A. Tarabees5
1Department of Geosciences, Hamilton College, Clinton, New York 13323, USA
2Geology and Geophysics Program, Missouri University of Science and Technology, Rolla, Missouri 65409, USA
3Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA
4Department of Geology, University of Vermont, Burlington, Vermont 05405, USA
5Department of Geology, Damanhour University, Damanhour 22516, Egypt
ABSTRACT
Although polygonal fault systems and related features are common in fi ne-grained sedi-
ments in modern submarine basins and have been studied in basins worldwide using three-
dimensional (3-D) seismic data, extensive on-land exposures have remained elusive. We
report here on the discovery of a polygonal fault system occurring in nearly continuous sur-
face exposure over ~900 km2 in chalk of the Cretaceous Khoman Formation near Farafra
Oasis, Egypt. Field exposures reveal polygon boundaries defi ned by clusters of dozens of
normal faults with strongly grooved fault surfaces and coarse calcite veins along faults with
evidence for multiple fl uid fl ow events. Geometric patterns and fault intersections reveal
that mechanically interacting normal faults with multiple orientations were active contem-
poraneously in a horizontal strain fi eld that was essentially isotropic and extensional. We
interpret the very steep dips (~80°) to refl ect fault initiation in response to elevated pore
fl uid pressures. In the uppermost part of the Khoman Formation, a terrain of isolated cir-
cular structures displaying shallow inward dips overlies the polygonal fault network. The
spatial relationship to the underlying faults is consistent with these small circular basins
having formed as fl uid escape structures as the polygonal fault system evolved. Outcrops in
the Khoman Formation provide an unprecedented look into the 3-D geometry of a polygo-
nal fault system, providing context for the analysis of analogous systems in marine basins
and other on-land exposures.
GEOLOGY, June 2014; v. 42; no. 6; p. 479–482; Data Repository item 2014165
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doi:10.1130/G35362.1
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Published online 15 April 2014
© 2014 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Farafra
Anticine
28.0°E 28.3°E
27.0°N 27.15°N
Legend
N
Waddi Hennis,
El Hefhuf Fms.
sand
20 km
Thebes Gp.
Cretaceous Paleogene
Esna Shale
Tarawan Fm.
Dakhla Shale
Khoman Fm.
areas of
polygonal faults
areas of small
circular basins
Fig 3A
inset
Fig 3B
Fig 3A
Fig 2
QF
Egypt
Figure 1. Geologic map showing areas of Khoman Formation that display polygonal
faults and small circular structures. QF—Qasr Farafra. Regional geology is adapted from
Klitzsch et al. (1987). Inset shaded relief map was developed from Shuttle Radar Topog-
raphy Mission DEM data.
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GEOLOGY
low fault dips in more deeply buried systems
(Cartwright, 2011). Berndt et al. (2003) argued
for long-term fl uid fl ow over millions of years in
the Vøring Basin, with multiple events related to
progressive evolution of the underlying polygo-
nal fault system. Polygonal faults are potential
fl uid conduits (Cartwright et al., 2003), and fl uid
escape pipes and seafl oor pockmarks have been
documented in sequences both within (Andre-
sen and Huuse, 2011) and overlying polygonal
faults (e.g., Berndt et al., 2003; Gay et al., 2004).
The mechanism of polygonal fault formation
remains unclear. Early models involving density
inversion, syneresis, and low coeffi cients of slid-
ing friction along fault planes (e.g., Cartwright
et al., 2003; Goulty, 2002) have been joined by
models invoking diagenesis (e.g., Davies et al.,
2009; Shin et al., 2008; Cartwright, 2011). To
date, no consensus model has emerged.
FAULT NETWORK IN THE KHOMAN
FORMATION
Our study area lies in massive white chalk
of the Late Cretaceous Khoman Formation, ex-
posed in the core of the broad regional Farafra
anticline (Fig. 1). The low relief of the wide, fl at-
fl oored Farafra Valley coupled with extremely
low limb dips (2°–3°) of the Farafra anticline
(Omara et al., 1970) combine to create wide-
spread bedding surface exposures over hundreds
of square kilometers. The chalk is fi ne-grained,
soft to medium-hard, and moderately consolidat-
ed with no clay insoluble residue, siliceous mi-
crofossils or macrofossils, quartz veins, or chert.
Exposures of the chalk display narrow resis-
tant fi ns 10–50 cm high, consisting of calcite
veins and vein complexes developed along faults
with steep dips (70° to nearly 90°) (Figs. 2A and
2B; see also Figs. DR1–DR3 in the GSA Data
Repository1). The host chalk is strongly grooved
along the fault surfaces (Fig. 2C), and rakes of
80°–90° indicate dip slip. Fault dip direction
combined with rare offsets of planar features
indicates normal slip (2–3 m calculated for the
fault in Fig. 2D).
Veins range in thickness up to 25 cm and
consist of coarse calcite crystals both parallel
and perpendicular to fault surfaces, commonly
extending into dilatant space. The veins create
casts of preexisting strongly grooved fault sur-
faces in the chalk (Fig. 2E; Figs. DR4 and DR5).
Calcite veins commonly exhibit multiple phas-
es, some with slivers of chalk between veins.
Damage zones within the host chalk are narrow
(5–10 cm) and are characterized by fractures of
multiple orientations fi lled by thin calcite veins.
The chalk also hosts very thin (1–2 mm) pla-
nar goethite veins that lack grooves and slick-
enlines and that were likely originally iron
sulfi de (pyrite or marcasite). Iron sulfi de (now
goethite) also occurs as crystal clusters, veneers,
and masses within some calcite veins. Isotopic
analyses from a small number of calcite vein
and host rock samples suggest reequilibration of
formation-generated (marine calcite) fl uids with
isotopically light meteoric waters (for methods
and data, see the Data Repository).
The overall organization of faults is spec-
tacularly revealed in high-resolution satellite
images. Faults are arranged in clusters of 5–20
quasi-parallel normal faults and fault segments
that defi ne polygonal areas 500–1000 m across,
with fewer faults in the polygon cores (Figs. 2F
and 2G; Fig. DR6). Faults show no regional pre-
ferred orientations (Fig. 2H), although rare faults
cut across multiple polygons (arrow, Fig. 2G).
Polygon corners are complex interaction zones
containing multiple fault orientations with in-
consistent crosscutting relationships that imply
overlap in the timing of their growth (Fig. 2I;
Fig. DR7), as well as arcuate faults that gradu-
ally curve from one side into an adjacent side of
the polygon. These features imply simultaneous
activity of normal faults with distinctly different
strikes and no dominant extension direction.
The polygonal network of faults in the Far-
afra Valley is confi ned to the Khoman Forma-
Fig. 2I
down wind
a
c
b
NW fault set
NE fault set
curving
faults
EW fault set
curving faults
100 m
N
NNW fault set
interaction zone
between NNW
and NE faults
N/SW
4000
2000
E
No. of 25 m
segments
CBA
DGF
H
I
E
1GSA Data Repository item 2014165, supplementary methods, and Figures DR1–DR11, is available online at www.geosociety.org/pubs/ft2014.htm, or on request from
editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
Figure 2. A–C: Low,
strongly grooved, coarse
calcite veins mark faults
in chalk. Rock hammer in
C is 27 cm long. D: Throw
calculated from fi eld mea-
surements (at stars) on
prominent fault in polygon-
bounding cluster is 2–3 m.
E: Multiphase veins with
calcite crystallized in dilat-
ant space, forming casts
of grooves along fault sur-
faces. Rock hammer head
is 9 cm long. F: Faults are
arranged in clusters that
border polygonal areas
with lower fault density.
Faults appear to be promi-
nent ridges in satellite im-
agery (illumination from
southeast). Field expo-
sures (in A) show that this
is an illusion produced by concentration of dark surface lag on upwind (northwest) sides of low calcite
veins. G: Fault traces mapped using satellite imagery show polygonal plan view of fault clusters. Rare
faults cut across multiple polygons (red arrow). H: Azimuths of faults in representative 25 km2 area
mapped on satellite imagery and divided into 25 m segments. I: Faults in north-northwest, northeast,
and northwest sets (defi ning individual polygon sides) commonly curve from one polygon side into
an adjacent one. Multiple fault orientations occur where polygon sides meet and interact, with incon-
sistent crosscutting patterns related to overlap in timing of fault growth. Insets: Faults a, b, and c
were each infl uenced by the others in terms of mechanical interactions (curving of one fault into an-
other) and crosscutting, implying overlap in timing of growth of each fault orientation. Image centers:
D—27.178449N, 27.965212E; F—27.160982N, 28.015432E; G—27.175447N, 28.010321E; I—27.160073N,
28.020536E. Images D, F, and I are DigitalGlobe WorldView I satellite images (www.digitalglobe.com/).
Also see Figures DR1–DR7 (see footnote 1).
GEOLOGY
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June 2014
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www.gsapubs.org 481
tion, which is at least 220 m thick (Barakat and
Hamid, 1974). The faults die out almost entirely
in the uppermost part of the Khoman Formation
and are absent in the underlying El Hefuf and
Wadi Hennis Formations (Fig. 1).
SMALL CIRCULAR BASINS
The upper part of the Khoman Formation is
characterized by more than 2000 isolated small
circular basins 50–200 m in diameter, spaced
200–500 m apart, and with inward dips of 5°–
20° (Figs. 1 and 3). The circular basins highest
in the section form small mesas <10 m high
capped by an inward-dipping resistant lime-
stone layer (Fig. 3A; Fig. DR8). Basins slightly
lower in the section lack both resistant caprock
and topographic relief. Some low-relief basins
occur in strong spatial association with poly-
gon boundaries and triple junctions (Fig. 3B;
Fig. DR9), are cut by normal faults, and display
radial vertical veins of calcite and goethite (orig-
inally pyrite or marcasite) (Fig. DR10).
Figure 3C shows the spatial relationship
among faults, low-relief basins, and basin-capped
mesas (Fig. DR11). Most of the small circular ba-
sins lie stratigraphically above the fault network,
although rare faults with grooved, multiphase
calcite veins extend upward through the basin-
capped mesas, which are highest in the section.
DISCUSSION
We propose that the Khoman Formation dis-
plays the fi rst extensively exposed occurrence of
polygonal faults and related fl uid fl ow features
to be described on land. Faults are layer-bound
normal faults clustered in a linked polygonal
network at scales typical for polygonal faults
(Cartwright, 2011) and formed in a strain fi eld
with no dominant extension direction. We also
propose that the small circular basins overlying
the polygonal fault system are fl uid escape fea-
tures related to polygonal faulting. The basins
are similar in size (100–200 m) to fl uid escape
pipes and seafl oor pockmarks that overlie po-
lygonal fault systems (e.g., Berndt et al., 2003;
Gay et al., 2004) and that have been attributed
to long-term fl uid fl ow and vertical stacking of
pockmarks as sedimentary layers accumulate
(Moss and Cartwright, 2010).
Our observations suggest an intimate rela-
tionship between fl uids and polygonal faults in
the Khoman Formation, supported mechanical-
ly using Mohr-Coulomb failure considerations.
The very steep fault dips (~80°) indicate that
shear failure occurred under conditions where
the Mohr circle intersected the failure envelope
where σ3 was negative (tensile), but had not
reached the tensile strength of the rock. This
condition was most likely achieved by driving
a small Mohr circle (with maximum shear stress
less than the rock cohesion) to the left by in-
creasing pore fl uid pressure. In contrast, a σ3 de-
crease caused solely by syneresis or diagenetic
volume decrease with a constant σ1 would have
resulted in an expanding Mohr circle intersect-
ing the failure envelope at positive σ3, unless σ1
were very small. Resultant faults would have
dips of ~60°, which is not what we observe.
Elevated pore fl uid pressure played a promi-
nent role in early models for polygonal fault
initiation (e.g., Henriet et al., 1991; Cartwright,
1994; Cartwright and Lonergan, 1996). Re-
cent attention has shifted to diagenetic models
to account for the signifi cant volumetric strain
involved in polygonal fault formation (e.g., Da-
vies et al., 2009; Shin et al., 2008; Cartwright,
2011). The absence of any evidence for silica
and clay in the Khoman Formation, however, in-
dicates that a model of fault initiation by volume
loss due to diagenesis of biogenic silica (Davies
et al., 2009) or smectite (Cartwright, 2011) is
neither applicable nor necessary to account for
polygonal faulting. For polygonal faults in the
Khoman Formation, we therefore conclude that
pore fl uid pressure played an important role
in fault initiation and subsequent activity. The
origin of these fl uid pressures is unknown, but
could reasonably be linked to the compaction
and induration of sediments during burial.
Once faults formed, repeated fl uid fl ow events
clearly utilized the faults as conduits, forming
vein systems. Although we have no unequivocal
evidence for the timing of fl uid fl ow and vein
mineralization, our isotopic data permit vein
formation as polygonal faults evolved (see the
Data Repository). One possibility is that cre-
ation of steep fault surfaces enabled subsequent
slip events to occur under hybrid dilational/shear
conditions, with euhedral calcite crystallizing in
dilational space as casts of grooved fault sur-
faces. As each slip event dilated the fault further,
new veins would have formed alongside older
ones. Alternatively, all veins may have postdated
faulting, with high fl uid pressures causing epi-
sodic dilation and vein formation along existing,
steep fault surfaces. Whichever timing relation-
ship is correct, simultaneous dilation occurred
in multiple directions, evidenced by numerous T
intersections along fault and vein systems.
Small circular basins cut by polygonal faults
in the Khoman Formation, along with sets of
basins with strong spatial correlation to polygon
boundaries and junctions (Fig. 3C), suggest that
polygonal faults grew upward over time through
basins formed earlier. The radial geometry of
veins around some of the low-relief circular ba-
sins spatially associated with polygonal faults
(Fig. 3B) is consistent with radial fracturing
caused by high transient fl uid pressures around a
fl uid escape pipe in a horizontally isotropic stress
fi eld. If true, this relationship may favor a model
in which high fl uid pressures and fl ow events
along faults occurred coeval with fault activity.
CONCLUSIONS
The Khoman Formation hosts extensive ex-
posures of polygonal faults and related fl uid
fl ow features, with macroscopic characteristics
compatible with polygonal fault systems known
from the marine environment. Our fi eld obser-
vations reveal small-scale features that were
AB
C
Khoman
Fm.
El Hefhuf Formation
~1000 m
~200 m
polygonal faults
small circular basins
Figure 3. A: Small mesas capped by inward-dipping layers. Inset shows mesa profi le. B:
Low-relief circular structures with inward-dipping layers and radial rays of calcite and
iron oxide (originally iron sulfi de) veins; circular structures are spatially associated with
polygon-bounding fault clusters (arrows). C: Schematic representation of faults and small
basins in Khoman Formation at current level of erosion. Image centers: A—27.289982N,
28.491490E; inset is located at 27.230642N, 27.930565E; B—27.221490N, 27.976172E. Im-
ages A and B are DigitalGlobe WorldView I satellite images (www.digitalglobe.com/). Also
see Figures DR8–DR11 (see footnote 1).
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GEOLOGY
previously unknown, such as tight fault clusters
within contemporaneously developing polygon
arms, extensive vein systems, and slip indica-
tors. Steep fault dips and abundant vein systems
suggest an intimate relationship between pres-
surized fl uids and faulting during both initial
mechanical failure of the chalk and the subse-
quent evolution of the system. Our fi eld observa-
tions provide new insights into these polygonal
fault systems in marine environments and may
assist in the recognition of on-land exposures
elsewhere.
ACKNOWLEDGMENTS
This study was supported by funding from the
National Science Foundation (grants 1030224 and
1030230) and Hamilton College (New York, USA).
Imagery was supplied by the Polar Geospatial Cen-
ter (Minnesota, USA). We gratefully acknowledge
mapping by Joseph Coons, Eric Doubet, David
Saint-Jacques, and Carolyn Tewksbury-Christle, and
isotopic analyses by Anthony Haigh and Jacob Vin-
cent, and Andrea Lini of the University of Vermont
Environmental Stable Isotope Laboratory. Careful
reviews and thoughtful commentary by editor Rob-
ert Holdsworth and reviewers Ian Alsop, Joseph
Cartwright, and Mads Huuse greatly improved the
manuscript.
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Manuscript received 8 December 2013
Revised manuscript received 2 March 2014
Manuscript accepted 4 March 2014
Printed in USA
