Figure Captions
Figure 1. Generalized fault framework of the area encompassing the
Nemaha zone, which lies between, and is terminated by, two
northwest-trending, left-lateral megashears, the Central Plains (to the
north) and the Oklahoma (to the south). En echelon, northwest-southeast
surface faults east of the Nemaha zone in Oklahoma form north- to
north-northeast-trends (Miser, 1954). The Nemaha zone and the other
fault trends are sites of many productive structures, which originated
by strike-slip movements. Trace of the Nemaha fault adapted from Berendsen and Blair (1992).
L= Lincoln; KC= Kansas City; OC= Oklahoma
City; T= Tulsa; W= Wichita.
Figure
2. Structural contour map, on top of eroded Precambrian surface, of the
area encompassing the Nemaha zone. The west-dipping, gentle homocline
over much of eastern Kansas is little disturbed by the Nemaha zone.
Contours in Kansas and Nebraska modified from Burchett et al. (1981);
Nemaha fault trace in Kansas adapted from Berendsen and Blair (1992).
Figure
3. Structural contour map, on base of Upper Pennsylvanian Kansas City
Group, of the area encompassing the Nemaha Zone.
Also at this level, the
Nemaha zone had minimal structural effect on the regional west-dipping homocline. Contours in Kansas, Oklahoma, and Nebraska modified from
Burchett et al. (1981). Nemaha fault trace in Kansas adapted from Berendsen and Blair (1992).
Click to view sequence of structural contour maps on top Precambrian and
Upper Pennsylvanian.
Figure
4. Structural contour map, on top of eroded Ordovician, of the giant El
Dorado oil field, in Butler County, Kansas (from Reeves, 1929). Contour
interval 50 feet. It is a typical transpressional (“pop-up”) structure,
about 15 miles long and 9 miles wide. It is bounded on the east by the
Nemaha fault , which was not mapped—following the accepted procedure for
contouring at the time this map was prepared. “Cherokee” (Middle
Pennsylvanian) strata overlie the eroded Cambro-Ordovician Arbuckle.
Figure
5. Structural contour map on top of Mississippian “lime” of the Garber
area, north-central Oklahoma, showing a pop-up block and an associated
graben. Every well (indicated by small dots) on this map has a value for
the top of eroded Mississippian, providing very detailed control for
delineating the fault pattern. The fault on the east is a
down-to-the-east reverse fault . The stratigrahic section in the graben
includes more than 600 feet of Lower Pennsylvanian and Upper
Mississippian strata that were stripped from both flanking upthrown
blocks.
Figure
6. Interpreted west-east geo-seismic cross-section across the Garber
structure. Most of the strike-slip movement is thought to have caused
the early reverse separation. Subsequent normal displacement resulted in
formation of the present trap . Line of section shown in
Figure 5.
Figure
7. Structural cross-section across the Garber graben. The interval
Pennsylvanian Hogshooter - top Mississippian “lime” is about 840 feet
thicker in the two fault blocks comprising the graben than in the areas
to the west and to the east. Line of section in
Figure 5.
Figure
8. Pre-Pennsylvanian subcrop map of the Oklahoma City uplift (from
Gatewood, 1970). This structure is a highly upthrown, pop-up reverse
fault block bounded on the east by a fault of the Nemaha zone. Maximum
throw is 2400 feet along a very straight, normal-vertical-reverse,
down-to-the-east fault .
Figure
9. Well-log structural cross-section in the Jennings, Oklahoma, area,
along the en echelon Whitetail fault trend (Figure 1),
showing a narrow graben that is known to have developed during parts of Middle and Late
Pennsylvanian. The Desmoinesian interval is 25% (100 feet) thicker than
it is on the upthrown block on the west. Numbers along the faults give
cumulative stratigraphic displacements, which decrease upward. Vertical
exaggeration required to illustrate detail.
Figure
10. Structural contour map, on top of pre-Pennsylvanian strata, at
Cushing field, south of the Jennings feature and on the same Whitetail
strike-slip fault trend (modified from Weirich, 1929). It is a high
relief, pop-up structure with essentially the same history as the
Oklahoma City, Garber, and El Dorado structures, and with Middle
Pennsylvanian strata overlying deeply eroded Arbuckle on its crest. It
should be noted that in 1929, when this map was published, geologists
did not recognize the role of high-angle faults.
Figure
11. Structural cross-section of graben in the Wetumka area, east-central
Oklahoma, located southeast and basinward of Jennings and Cushing; it is
on an en echelon fault trend east of the Whitetail trend. Desmoinesian
strata in the graben are thicker than on the flanks due to syntectonic
deposition. Vertical exaggeration required to illustrate detail.
Regional Setting
The Nemaha zone trends north-northeast through Kansas and north, with
westward convexity, through most of Oklahoma. It is bounded on the north
by the Central Plains Megashear and on the south by the Oklahoma
Megashear (Figure 1), composed of major faults of the Arbuckle and
Amarillo-Wichita-Criner Hills uplifts. Both megashears are
northwest-trending and left-lateral, and episodic movements along them
probably extended from Middle Proterozoic to Late Paleozoic.
The area east of the Nemaha zone in Oklahoma exhibits several north- to
north-northeast-trending strings of en echelon surface faults, which are
oriented northwest-southeast (Miser, 1954). These are thought to be
manifestations of strike-slip faults, along which are many oil- and
gas-bearing structures.
At the top of the Precambrian basement and also at the level of Upper
Pennsylvanian strata, the west-dipping, gentle homocline over much of
eastern Kansas is split, but little disturbed, by the Nemaha zone
(Figures 2 and 3), even though some of the pop-up horst-blocks in the
zone display over 1600 ft of vertical displacement. At the Nemaha uplift
in northeastern Kansas, the fault is high-angle, reverse, and
down-to-the-east with 2500 ft of throw (Carlson 1970). Across most of
Kansas, the fault is generally down-to-the-east. Many of the
horst-blocks in southern Kansas were elevated sufficiently for
post-Mississippian erosion to remove over 1000 ft of Lower Paleozoic
strata and expose the Precambrian basement, which was later overlain by
Middle and Upper Pennsylvanian strata. Near the Oklahoma line, vertical
displacement decreases and reverses to down-to-the-west. This feature is
typical in Oklahoma, except in a few local areas where highly elevated
pop-up structures are associated with down-to-the-east reverse faults.
Examples of Local Structures within Nemaha Zone and Associated Fault
Trends
El Dorado Oil Field
The giant El Dorado oil field, in Butler County, southeastern Kansas,
was discovered in 1915 as a surface feature. It is about 15 miles long
and 9 miles wide, bounded on the east by the Nemaha fault (Figure 4).
This feature apparently formed as a transpressional (pop-up) structure
during the post-Mississippian hiatus when several thousand feet of
strata were eroded. Reeves (1929) noted that approximately 1400 feet of
“Siliceous Lime” (Cambro-Ordovician Arbuckle) were removed. “Cherokee”
(Middle Pennsylvanian) strata overlie eroded Arbuckle. The field has
produced over 400 million barrels of oil.
Garber Field Area
The Garber area, north-central Oklahoma, is characterized by a pop-up
block with an associated graben (related to a releasing bend or
representing a pull-apart “basin”) (Figure 5). This area is thought to
offer the best evidence for right-lateral strike-slip movement along
faults of the Nemaha zone. By Early Pennsylvanian, when there was
erosion of the Garber structure, the stratigraphic throw, based on the
top of the Mississippian “lime,” could have been as much as 3100 feet.
Over 1000 ft of strata, ranging in age from Late Mississippian to Middle
Ordovician, were removed by post-Mississippian erosion. The top of the
Mississippian “lime” in the graben is more than 1600 feet lower than it
is on the eroded upthrown east block and over 900 feet below it in the
west block. The east-bounding fault is a reverse fault ,
down-to-the-east. The graben has preserved in it more than 200 feet of
Lower Pennsylvanian clastics and over 400 feet of Upper Mississippian
Chesterian oil-bearing strata that was stripped from both upthrown
flanking blocks during Early Pennsylvanian erosion. This graben had been
known for nearly forty years, but condemned by the industry because it
was “low,” before a detailed study resulted in the completion of ten
wells with new reserves in the Manning zone of the preserved Chesterian
interval in the graben. To the north and south of the Garber structure,
the net vertical displacement is down-to-the-west.
Most of the
strike-slip movement is thought to have occurred in Pennsylvanian time,
when fault separation was reverse (Figure 6). Normal separation occurred
in the Permian, with arching of the strata over the fault block and
thereby forming the present trap for the more than 65 million barrels of
oil that have been produced since 1949.
The interval from
the Pennsylvanian Hogshooter to the top of the Mississippian “lime” is
about 840 feet thicker in the graben than in the upthrown blocks to the
west and east (Figure 7). The earliest identifiable movement along the
Nemaha zone was in Middle Ordovician time, as indicated by a 25% increase
in the thickness of the Viola Limestone in the Garber graben, compared to
the upthrown flanks. Its latest movement was in Middle Permian time.
Oklahoma City Field
The Oklahoma City uplift has a structural history, geometry, and
position similar to that of Garber and El Dorado. It is a highly
upthrown, pop-up, reverse fault block that formed within the Nemaha
zone. The east-bounding fault is down-to-the-east (Figure 8), although
to the north and south, the relative displacement is down-to-the-west.
Post-Mississippian erosion removed approximately 2000 feet of
Mississippian, Devonian, Silurian, and Ordovician strata. Upper Middle
Pennsylvanian deposits overlie eroded Arbuckle over the crest of the
structure. According to Gatewood (1970), maximum throw of the Oklahoma
City fault is 2400 feet, and the fault is very straight, generally
normal or vertical, but apparently high-angle reverse in some sectors.
Discovered in 1926, the field has produced over a billion barrels of
oil. South of the
Oklahoma City uplift, the Nemaha zone is accentuated with the
development of several north-northeast- to northwest-trending vertical
faults with very large down-to-the-west displacements. This pattern for
the Nemaha zone continues to the south, where it splays-out against the
Oklahoma Megashear. The Nemaha faults were contemporaneous with rapid
subsidence of the Anadarko basin immediately to the west; both elements
during the Pennsylvanian Desmoinesian were affected by strong movements
along the Oklahoma Megashear. This area south of Oklahoma City contains
the fabulous Golden Trend, which will produce well over 500 million
barrels of oil from strata of mid-Desmoinesian and Ordovician age,
mostly trapped against the Nemaha faults.
Jennings Oil Field
The Jennings area, northeastern Oklahoma, is situated along the en
echelon Whitetail fault trend east of the Nemaha zone (Figure 1). The
structure includes a severely faulted graben, which is bounded by
left-lateral faults (Figure 9). The Desmoinesian interval is 100 feet
(25%) thicker in the graben than it is in the west-flanking block.
Movements along the fault during the Pennsylvanian probably occurred
over a period of 5-10 million years. Jennings, like the Garber area,
represents a case where the structure (in the graben) was known to be
low and correspondingly was “condemned” for 30 years before the
development of eight new wells that resulted in additional reserves.
Cushing Field
Cushing field, south of the Jennings feature, is on the same Whitetail
strike-slip fault trend. The controlling fault bounds the structure on
the east and is down-to-the-east (Figure 10). It is a high relief,
pop-up structure with essentially the same history as the Oklahoma City,
Garber, and El Dorado structures, and with Middle Pennsylvanian strata
overlying deeply eroded Arbuckle on its crest. Similar to the map of the
El Dorado structure (Figure 4), the fault was not shown by Weirich
(1929) as part of the contouring procedure (Figure 10). This field has
produced more than 600 million barrels of oil since its discovery as a
surface feature in 1916.
Wetumka Area
The Wetumka area, east-central Oklahoma, is located southeast of
Jennings and Cushing on an en echelon fault trend east of the Whitetail
trend. It is very similar to Jennings in that a narrow graben formed as
a part of the overall structure (Figure 11). The Pennsylvanian sequence
here is considerably thicker than at Jennings due to its more basinward
position. Desmoinesian strata in the graben are about 15% thicker than
on the flanks due to deposition during fault movement.
Conclusions
The chronology and geometry of the Nemaha fault zone is well documented
by data from the hundreds of wells drilled in oil fields situated in and
adjacent to the zone in Kansas and, especially, in Oklahoma. The
earliest known movement of the Nemaha zone occurred during the
Ordovician, although it may have originated at the same time as its
terminating faults, i.e., in Middle Proterozoic.
Along its entire length, the Nemaha fault surfaces are vertical or
near-vertical, as determined from many closely spaced wells and seismic
data. Along its trace, the directional sense of throw is reversed in
several segments. Additional evidence for a major strike-slip fault zone
consists of the presence of restraining and releasing bends (pop-up
horsts and pull-apart grabens, with syntectonic stratal thickening,
respectively) and splaying at both ends into its terminating megashears.
The Nemaha zone is the result primarily of transpressional stresses
acting episodically over a very long time. Right-lateral, strike-slip
movement is thought to be especially well documented in the Garber field
area.
For comparison,
transpressional horst blocks, transtensional grabens, and numerous large
fields are common along the long, narrow Matador zone in north-central
Texas. This zone is regarded as a left-lateral fault (Brister et al.,
2002).
Berendsen, P., and K.P. Blair, 1992,
Precambrian structure map (of Kansas): Kansas Geological Survey, Open
file report 92-41 A.
Brister, B.S., W.C. Stephens, and G.A.
Norman, 2002, Structure, stratigraphy, and hydrocarbon system of a
Pennsylvanian pull-apart basin in north-central Texas: AAPG Bulletin, v.
86, p. 1-20.
Burchett, R.R., K.F. Luza, O.J. Van Eck,
and F.W. Wilson, 1981, Seismicity and tectonic relationships of the
Nemaha Uplift and Midcontinent geophysical anomaly (final project
summary): Oklahoma Geological Survey Special Publication 81-82.
Carlson, M.P., 1970, Distribution and
subdivision of the Precambrian and Lower and Middle Paleozoic rocks in
the subsurface of Nebraska: Nebraska Geological Survey Report of
Investigations no. 3, 25 p.
Miser, H.D., 1954, Geological Map of
Oklahoma: Oklahoma Geological Survey.
Reeves,J.R., 1929, El Dorado Oil Field,
Butler County, Kansas, in Structure of typical American oil
fields, v. 2, AAPG, p. 160-167.
Weirich,
T.E., 1929, Cushing oil and gas field, Creek county, Oklahoma, in
Structure of typical American oil fields, v. 2, AAPG, p. 396-406.
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