Precambrian Basement Geology of the Permian Basin Region of

Precambrian Basement Geology of the Permian Basin
Region of West Texas and Eastern New Mexico:
A Geophysical Perspective1
Donald C. Adams and G. Randy Keller2
ABSTRACT
Because most of the Permian basin region of
west Texas and southern New Mexico is covered
by Phanerozoic rocks, other means must be found
to examine the Precambrian upper crustal geology
of the region. We have combined geologic information on the Precambrian from outcrops and wells
with geophysical information from gravity and magnetic surveys in an integrated analysis of the history
and structure of basement rocks in the region.
Geophysical anomalies can be related to six
Precambrian events: formation of the Early
Proterozoic outer tectonic belt, igneous activity in
the southern Granite-Rhyolite province, an episode
of pre-Grenville extension, the Grenville orogeny,
rifting to form the Delaware aulacogen, and
Eocambrian rifting to form the early Paleozoic continental margin. Two geophysical features were
studied in detail: the Abilene gravity minimum and
the Central Basin platform gravity high. The
Abilene gravity minimum is shown to extend from
the Delaware basin across north-central Texas and
is interpreted to be caused by a granitic batholith
similar in size to the Sierra Nevada batholith in
California and Nevada. This batholith appears to be
related to formation of the southern GraniteRhyolite province, possibly as a continental margin
arc batholith. Because of this interpretation, we
have located the Grenville tectonic front southward from its commonly quoted position, closer to
the Llano uplift. Middle Proterozoic mafic intrusions are found to core the Central Basin platform
©Copyright 1996. The American Association of Petroleum Geologists. All
rights reserved.
1Manuscript received November 9, 1994; revised manuscript received
July 31, 1995; final acceptance October 2, 1995.
2Department of Geological Sciences, University of Texas at El Paso, El
Paso, Texas 79968-0555.
We would like to acknowledge the help and assistance provided through
discussions with R. E. Denison (University of Texas at Dallas), Ronald
Broadhead (New Mexico Bureau of Mines), Calvin Barnes (Texas Tech
University), Kent Neilson (University of Texas at Dallas), and Elizabeth
Anthony (University of Texas at El Paso). The reviews provided by Sharon
Mosher and William Wilbert helped us significantly improve the manuscript.
The support of our rift-related studies by Conoco is also greatly appreciated.
410
and the Roosevelt uplift. These intrusions formed
at about 1.1 Ga and are related in time to both the
Mid-Continent rift system and the Grenville orogeny in Texas. Because these features are likely to be
rift related, they suggest that the concept of a
Delaware aulacogen needs to be revised only to the
extent that the rifting is Proterozoic in age, not
Eocambrian. Precambrian basement structures and
changes in lithology have influenced the structure
and stratigraphy in the overlying Permian basin,
and thus have potential exploration significance.
Interpretation of the gravity and magnetic data
with geologic information also leads us to suggest
the existence of pre-Ellenburger basins, which may
be extensive and of potential exploration interest.
INTRODUCTION
The Precambrian basement provides the structural framework for most continental areas, and
Precambrian sedimentary rocks have proven to be
viable exploration targets in the search for hydrocarbons. In addition, evidence continues to
mount on the degree to which old structures control the nature and location of younger ones.
However, large Phanerozoic basins, which are
important exploration targets by their nature,
cover the Precambrian basement, obscuring this
important part of the rock record. Thus, we must
rely on data from existing drill holes and geophysics to understand the basement in such areas.
The Permian basin of west Texas and eastern New
Mexico (Figure 1) is one of North America’s major
basins from both a geologic and a petroleum perspective. Although tens of thousands of wells have
been drilled in this basin, only about 2000 have
encountered the Precambrian basement, and most
of these wells are concentrated on basement
highs such as the Central Basin platform and the
Matador arch (Figure 1). In addition, with few
exceptions, these basement penetrations are
only a few feet. Thus, even in this well-known
basin, our knowledge of the Precambrian basement is limited; the purpose of this paper is to begin
AAPG Bulletin, V. 80, No. 3 (March 1996), P. 410–431.
Adams and Keller
411
Figure 1—Location map
showing the positions of
the outer tectonic belt
(OTB), the Swisher– (ST)
Debaca (DT) terrane, the
southern Granite-Rhyolite
province (SGRP), the
Grenville province (GP),
and the Carrizo group
(CG). The locations of the
mafic bodies associated
with the Central Basin
platform (CBP), the west
platform fault (WPF), the
Roosevelt uplift (RU),
Pajarito Mountain (PM),
and the Crosbyton
geophysical anomaly (C)
are shown. The locations
of Precambrian outcrops
at Van Horn (VH), Pump
Station Hills (PS), Hueco
Mountains (HM), Pajarito
Mountain (PM), and
Pedernal Hills (PH) are
shown. Thick black lines
mark the locations of
gravity models DD′, MM′,
NN′, and SS′. Finally, the
locations of wells
penetrating Precambrian
basement and the rock
types penetrated are also
shown. The (N) locates the
North American Royalties
1 Nellie well. Modified
from Flawn (1956), Roth
(1960), Denison et al.
(1984), Adams et al. (1993),
and Reed (1993).
to address this problem. In this study, we integrated gravity and magnetic data and models
with borehole and outcrop information to examine the geology and structure of the Precambrian
part of the upper crust. Two basement features,
namely the Abilene gravity minimum and Central
Basin platform, were singled out for detailed
examination through 2.5-dimensional gravity
modeling.
PREVIOUS STUDIES OF THE PRECAMBRIAN
BASEMENT
Our investigation expands on previous efforts to
combine outcrop and drilling information to produce maps of the geology of the Precambrian subcrop in this area (e.g., Flawn, 1956; Muehlberger et
al., 1967; Denison et al., 1984; Reed, 1993).
Previous maps of the Permian basin area show that
412
Precambrian Basement Geology
the Precambrian basement generally decreases in
age from 1.70 Ga in the northwest to 1.07 Ga in the
southeast. The oldest dated rocks are represented
by the Hondo group in the Pedernal Hills
(Robertson et al., 1993), and the youngest dated
rocks are represented by granitic intrusions in the
Llano uplift (Walker, 1992) (Figure 1). The results
of four tectonic events, namely the formation of
the outer tectonic belt (1.70–1.60 Ga), the formation of the southern Granite-Rhyolite province
(1.40–1.34 Ga) (Van Schmus et al., 1993b), the
Grenville orogeny (1.3–1.0 Ga) (Walker, 1992), and
periods of Middle Proterozoic extension (1.22–1.07
Ga) (Adams et al., 1993; Adams and Keller, 1994),
are recorded by the rocks in this area (Figure 1).
The outer tectonic belt is an orogenic terrane
that extends along the Early Proterozoic southeastern margin of North America from Michigan to
Arizona (Van Schmus et al., 1993a), and it is represented by outcrops in the Pedernal Hills (Figure 1)
(Robertson et al., 1993). The southern limit of the
outer tectonic belt may coincide with the Grenville
front (Van Schmus et al., 1993a), which is commonly drawn along the Abilene gravity minimum (e.g.,
Muehlberger et al., 1967; Denison et al., 1984;
Ewing, 1990; Reed, 1993) in Texas. This gravity
anomaly is a linear gravity feature that extends for
600 km between the Texas–Oklahoma border and
the Central Basin platform (Figure 1) (Logue, 1954).
Rocks of the outer tectonic belt may have provided the basement on which the southern GraniteRhyolite province was formed (Van Schmus et al.,
1993a). The southern Granite-Rhyolite province is
an area composed of 1.40–1.34 Ga [dated using
uranium/lead (U/Pb)] granite, rhyolite, and dacite
located in Arkansas, Missouri, Oklahoma, Kansas,
Texas, and New Mexico (Figure 1) (Thomas et al.,
1984; Van Schmus et al., 1993b). The rocks of this
terrane have been interpreted to have formed in
either an extensional (Bickford and Anderson,
1993) or a subduction-related (Patchett and Ruiz,
1989) tectonic environment.
Slightly younger rocks have been found south of
the southern Granite-Rhyolite province. Near Van
Horn, Texas, a 1327 ±28 Ma (U/Pb) rhyolite has
been found in the Carrizo Mountain group (Roths,
1993), and in the Llano uplift an igneous protolith
age of 1303 +5/–3 Ma (U/Pb) is reported for part of
the Big Branch Gneiss (Walker, 1992) and approximately 1.36 Ga (U/Pb) in the Valley Spring Gneiss
(Reese, 1993). These ages indicate the presence of
basement south of the Abilene gravity minimum
comparable in age to southern Granite-Rhyolite
province rocks north of it, possibly indicating a
relationship between them (Reese, 1993).
Evidence of Grenville-age tectonic activity consisting of complex multiple deformations and
igneous activity is found in the Llano and the Van
Horn uplifts (Figure 1). The Llano uplift is the
largest exposure of Precambrian rocks in Texas
(e.g., Mosher, 1993). The rocks in this area consist
of gneisses and schists which formed during the
Grenville orogeny and were intruded by granites
toward the end of the orogeny. U/Pb zircon dates
from the Llano uplift yield the protolith ages of
1.36–1.23 Ga for the metamorphic rocks and ages
of formation of 1.12–1.07 Ga for the Town
Mountain Granites. The main period of Grenvilleage tectonism in the Llano uplift probably occurred
between the youngest protolith age of the gneisses
and schists and the oldest Town Mountain Granite
(i.e., between 1.23 and 1.12 Ga).
In west Texas, rocks affected by Grenville-age
tectonism are found in the Van Horn uplift (e.g.,
Mosher, 1993). These rocks are referred to as the
Carrizo and Allamoore groups and are separated by
the Streeruwitz thrust. The Carrizo group rocks are
southeast of the thrust, and increase in metamorphic grade away from the thrust. The parent rocks
of the Carrizo group are rhyolites, sandstones, and
siltstones intruded by diabase sills. The environment of formation of the Carrizo group has been
variously interpreted as a back-arc spreading environment (Rudnick, 1983; Condie, 1986) or a rift
environment related to opening of a pre-Grenville
ocean (Roths, 1993). The Carrizo group was thrust
on top of the Allamoore group during Grenville-age
tectonism. The Allamoore group consists of carbonates and basalt flows (Mosher, 1993), which are
thought to be correlative with the Castner marble
and the Mundy Breccia in the Franklin Mountains
of far west Texas (Denison and Hetherington,
1969). Geochronology on ash layers in the
Allamoore Formation (1250 +16/–24 Ma) (Roths,
1993) and the Castner marble (1260 ±20 Ma)
(Pittenger et al., 1994) have shown this relationship to be true. The Castner marble is interpreted
to have been deposited along a subsiding continental margin associated with rifting or back-arc
spreading (Pittenger et al., 1994). It is generally
thought, without detailed confirmation, that the
Texas Grenville province extends in the subsurface
from the Llano uplift 300 km northward to the
Abilene gravity minimum (e.g., Muehlberger, 1965;
Denison et al., 1984; Mosher, 1993) (Figure 1). The
Abilene gravity minimum marks a transition in the
basement from mostly Granite-Rhyolite terrane
rocks in the north to metasedimentary rocks and
gneisses south of it (S. Mosher, 1995, personal communication).
During the period from 1.25 to 1.07 Ga numerous igneous intrusions were injected into the
Precambrian basement of west Texas and eastern
New Mexico (Figure 1). The intrusive bodies can
be divided into two groups based on whether they
formed before or after Grenville-age deformation of
Adams and Keller
the Llano uplift. The first group consists of igneous
rocks formed before the time of deformation and
includes the protoliths of the gneisses and schists
in the Llano uplift (Walker, 1992); the Carrizo and
Allamoore groups at Van Horn; the Mundy Breccia
in the Franklin Mountains (Denison and Hetherington, 1969); and possibly mafic rocks near
Pajarito Mountain in eastern New Mexico (Bowsher,
1991). The second group consists of intrusive
rocks formed during or after the late stages of
deformation and includes the Town Mountain
Granites in the Llano uplift (Walker, 1992); the
mafic rocks of the Central Basin platform (Keller
et al., 1989); the mafic rocks represented by the
Crosbyton geophysical anomaly (Adams and
Keller, 1994); the Red Bluf f Granite and the
Thunderbird rhyolite in the Franklin Mountains
(Roths, 1993); the mafic rocks in the Roosevelt
uplift; and possibly the Debaca–Swisher terrane
(Denison et al., 1984).
The igneous rocks in the Franklin Mountains, the
Central Basin platform, and the Van Horn uplift are
interpreted to be related to extension (Adams et al.,
1993). North America during this time appears to
have undergone an extensive period of bimodal
volcanism, including formation of the MidContinent rift system (Cannon, 1994), the Pikes
peak batholith (Anderson, 1983), mafic sills in
Arizona (Larson et al., 1994), California and Nevada
(Howard, 1991), and volcanism in New Mexico and
west Texas (Adams and Keller, 1994). The younger
group of intrusions is aligned along the trends of
both the Mid-Continent rift system and the mafic
sills in Arizona, California, and Nevada (Adams and
Keller, 1994).
Tweto (1983) interpreted the Las Animas
Formation in southeastern Colorado to be the sedimentary fill of a west-northwest–oriented rift basin.
The age of this basin is thought to be Late
Proterozoic because the basin is younger than the
underlying southern Granite-Rhyolite province but
older than the Middle Cambrian sediments that
overlie the basin. Based on geologic relationships
and compositional similarities, this basin is thought
to be the same age as the Tillman metasedimentary
group in Texas and Oklahoma (Tweto, 1983). Based
on the presence of a very ref lective basement
under the Hardeman basin area, Pratt et al. (1992)
suggested that the Tillman metasedimentary group
may underlie a large area of north-central Texas.
Keller and Baldridge (1995) point out that low gravity values in this region indicate that these rocks
have relatively low densities, which suggests that
they still have significant porosity.
The previous studies summarized above attest to
the complex history the Precambrian basement in
the Permian basin area has experienced, and to the
potential for Late Proterozoic basin development.
413
PREVIOUS GEOPHYSICAL STUDIES
Because the Permian basin in Texas and New
Mexico is an important petroleum basin, many geophysical investigations have examined the
Phanerozoic geology of the region. However,
because of the proprietary nature of this work,
only a few studies have been published. Keller et al.
(1985) provide a review of work up to that time. In
addition, geophysical studies of the Precambrian
basement are very rare.
In terms of overall crustal structure, seismic studies have found that the crust is approximately 50 km
thick near the New Mexico–Texas state line (Stewart
and Pakiser, 1962) and under the Central Basin platform (Kingwell, 1991). Kingwell (1991) also found
from seismic modeling that the crust appears to thin
eastward across the Permian basin and that the
lower crust has high velocities and densities. Surface
wave dispersion studies summarized by Braile et al.
(1989) show that this eastward decrease in crustal
thickness plateaus at about 40 km in central Texas.
On a more local basis, the possible presence of a
mafic body under the Central Basin platform was
first proposed by Keller et al. (1980). This study
used gravity modeling of profiles extending from the
Delaware basin onto the Central Basin platform near
the Texas–New Mexico state line. The gravity models were constrained in the Phanerozoic section by
density and lithology information from deep wells.
Modeling showed that additional mass under the
Central Basin platform was necessary to account for
the gravity high associated with it. The missing mass
was identified by the North American Royalties 1
Nellie well that was drilled into the gravity high of
the Central Basin platform, penetrating 4.5 km of
mafic rocks (Keller et al., 1989). Age determinations
on samples from this well show that the mafic intrusion formed between 1.16 and 1.08 Ma (U/Pb).
A deep seismic reflection study, the Hardeman
basin COCORP (Consortium for Deep Continental
Ref lection Profiling) seismic profile, is located
within the area of this study. The COCORP study
took place in Hardeman County, Texas, and
showed the presence of f lat-layered reflections
within the Precambrian basement (Brewer et al.,
1981). These reflections are thought to represent a
Middle Proterozoic basin possibly related to the
Tillman metasedimentary sequence. In southern
Oklahoma, these reflections are truncated on the
north by the Burch fault, which is the boundary of
the Wichita uplift (Brewer et al., 1981).
THE GEOPHYSICAL DATABASE
The gravity and magnetic data used in this study
were obtained from databases maintained by the
414
Precambrian Basement Geology
Figure 2—Residual gravity
anomaly map with a
second-order polynomial
removed. The map shows
the complexity of the
gravity field in the
Permian basin. Important
geological and geophysical
features are located on the
map: Delaware basin (DB),
Midland basin (MB), Val
Verde basin (VB),
Marfa basin (MRB), Central
Basin platform (CBP),
Pecos arch (PA), Abilene
gravity minimum (AGM),
Crosbyton anomaly (C),
and Roosevelt uplift (RU).
Depar tment of Geological Sciences at the
University of Texas at El Paso. The gravity data
consist of a set of 29,000 observations from which
simple Bouguer gravity values were calculated.
This data set was gridded at a 4-km spacing using
a minimum curvature algorithm. A second-order
polynomial surface was removed from the grid to
produce a residual gravity anomaly map (Figure
2), which we feel represents the crustal contribution to the complex gravity field. Gravity lows are
associated with parts of the Delaware, Midland,
Val Verde, and Marfa basins. Gravity highs are
associated with the Central Basin platform and
Pecos arch. Other features of the map such as the
Abilene gravity minimum and its flanking gravity
highs and the localized highs within the Delaware
basin are not clearly related to known Phanerozoic structures.
As the National Uranium Resource Evaluation
program (NURE) wound down, the University of
Adams and Keller
415
Figure 3—Residual magnetic
anomaly map with a
second-order polynomial
removed. This map
indicates changes in
magnetic susceptibility
(changes in the amount of
magnetite) in the
Precambrian basement.
nT = nanotesla.
Texas at El Paso and the Bureau of Economic
Geology of the University of Texas worked with
Bendix Field Engineering to ensure that the aeromagnetic data for Texas were compiled, tied
together, and edited to produce a high-quality
database. These data, along with a grid of magnetic
values for New Mexico (Cordell, 1984), were used
in this study. Both data sets consist of total field
magnetic data, with the appropriate international
geomagnetic reference field (IGRF) removed, and
were adjusted to a constant level. The magnetic
anomaly data were gridded to produce a grid that
is coincident with the gravity grid, and a secondorder polynomial surface was removed from the
grid to produce a residual magnetic anomaly map
(Figure 3) that primarily shows changes in susceptibility of the Precambrian rocks in the Permian
basin region.
416
Precambrian Basement Geology
Figure 4—Elevation of the
top of the Ellenburger
formation relative to sea
level, compiled from Galley
(1958) and Ewing (1990).
The top of this formation is
used as the base of the
gravity stripping model.
The following basin
features are located:
Central Basin platform
(CBP), Pecos arch (PA),
Matador arch (MA),
Delaware basin (DB),
Midland basin (MB),
Val Verde basin (VB),
Palo Duro basin (PDB),
Northwestern shelf (NWS),
and Diablo platform (DP).
PHANEROZOIC STRUCTURES AND
DENSITY UNITS
The gravity effects of Phanerozoic lithologic and
structural changes in the Permian basin must first be
accounted for to effectively study the Precambrian
geology. Phanerozoic sedimentation started in
Cambrian time with formation of the Tobosa basin
(Frenzel et al., 1988). Sedimentary rocks in the
Tobosa basin consist primarily of carbonates and
shales deposited in a tectonically quiet environment,
which lasted until the Late Mississippian. Late
Mississippian tectonic activity related to the
Ouachita orogeny produced large fault-bounded
structures and altered sedimentation patterns (e.g.,
Frenzel et al., 1988) in the basin (Figure 4). Foreland
deformation related to this orogeny caused the uplift
of the Central Basin platform, Pecos arch, and
Matador arch, thus breaking the Tobosa basin into
several subbasins, which include the Delaware,
Adams and Keller
417
(Table 1). The density units were determined by
the following method. The depths of formation
tops were determined for each well based on scout
ticket information and gamma-ray log signatures.
Next, contiguous formations having like densities
were grouped into density units and weighted averages were used to determine the density of each
unit in each well. Finally, like density units from all
wells were averaged to produce average densities
for the units in the basin.
GRAVITY STRIPPING ANALYSIS
Figure 5—Simplified stratigraphy of the Permian basin
region showing the tops and bottoms of density units
obtained from analysis of density logs in the Delaware
and Midland basins and on the Central Basin platform.
Densities for the Delaware basin and Central Basin platform are from Djeddi (1979). The Midland basin densities were determined in this study. Time gaps in the
stratigraphy are indicated by the diagonal lines.
Midland, Val Verde, and Palo Duro basins (e.g.,
Frenzel et al., 1988) (Figure 4). The basement uplifts
are located in areas of crustal weakness possibly
associated with Proterozoic structures (Hills, 1984).
These structures have up to 7.3 km (24,000 ft) of
relief and produce significant gravity anomalies. The
densities of the sediments were affected also by the
changes in sedimentation patterns (Figure 5).
Tectonic activity ended in the Permian, with sedimentation returning to that of a tectonically quiet,
slowly subsiding basin.
To account for changes in the density of sedimentary rocks in the Permian basin for gravity
modeling, formation densities were determined
from well logs. Djeddi (1979) determined densities
and density units for the Delaware basin and
Central Basin platform (Figure 5). This study determined densities of Midland basin rocks through use
of 13 density logs from wells in the Midland basin
The complexity of the gravity field of the
Permian basin (Figure 2) is caused by both
Phanerozoic features and the heterogeneity of the
underlying Precambrian basement. We would like
to separate the signals originating in the basement,
and one way of doing this is to apply the gravity
stripping method of Hammer (1963). By calculating the gravity effect of the Phanerozoic strata and
subtracting it from the observed gravity field, one
can produce a “deep source” gravity anomaly map.
This map would contain the part of the observed
anomalies that are unaccounted for by the model of
the Phanerozoic.
In our analysis of the Permian basin through
gravity stripping, we assumed that the sediments
between the surface and the top of the Ellenburger
formation could be lumped into a single unit having an average density of 2.55 g/cc. As Figure 5
indicates, this is a reasonable estimate of the bulk
density of these Permian basin strata. We chose the
top of the Ellenburger formation as the bottom of
the model because information on the geometry
and density of the formations below this level is
sparse and because the density of these rocks is not
appreciably different from that of the Precambrian
basement. The rocks below this level were
assumed to have a density that equates to that of
average metamorphic rocks (2.75 g/cc) (Telford et
al., 1984), producing a density contrast of –0.20
g/cc for the basin above the Ellenburger formation.
This value errs on the side of being too high, and
thus is conservative because it overestimates the
gravity anomaly caused by the basin. The result is
an underestimate of the contr ibution of the
Precambrian to the gravity anomalies. The residual
gravity grid (Figure 2), the surface topography grid,
and the elevation grid for the top of the Ellenburger
(Figure 4) were all calculated with a minimumcurvature algorithm to produce coincident grids
with a 4-km grid-point spacing. The topographic
data used in this study were taken from the elevations of gravity stations in the University of Texas at
El Paso database. The elevations of the top of the
Ellenburger formation were obtained by digitizing
418
Precambrian Basement Geology
Table 1. Wells Used for Density Determinations in the Midland Basin
County
Andrews County
Gaines County
Martin County
Midland County
Terry County
Upton County
Well
American Quasar Petroleum Co. 1–12 University, 660 FNL, 660 FEL, Sec. 12, Blk. 6, University Lands Survey
American Quasar Petroleum Co. 1–13 University, 440 FNL, 467 FWL, Sec. 13, Blk. 6, University Lands Survey
Gulf Oil Corp. 1 State “PG”, 1320 FSL, 1320 FEL, Sec. 24, Blk. 7, University Lands Survey
Amoco Production Co. 1 Arco-Mobil (OWWO), 990 FNL, 990 FEL, Sec. 21, Blk. H, D&W RR Survey
Amoco Production Co. 1-B Thornton Lomax Jr., 660 FSL, 660 FWL, Sec. 17, Blk. H, D&W RR Survey
James P. Dunnigan Inc. 1 K.C. Lawson et al., 853 FSL, 853 FWL, Sec. 23, Blk. 34 T-5-S, T&P RR Survey
Gulf Energy and Minerals 1-A G.W. Glass et al., 1320 FSL, 1320 FWL, Sec. 2, Blk. 39 T-1-N, T&P RR Survey
RK Petroleum 1-4 Scharbauer Ranch, 2173 FSL, 467 FEL, Sec. 4, Blk. 39 T-1-S, T&P RR Survey
Tamarack Petroleum Co. 1-32 Bradford, 1980 FSL 660, FWL, Sec. 32, Blk. 39 T-3-S, T&P RR Survey
NRM Petroleum Corp. 1-9 ODC, 1980 FSL, 1980 FWL, Sec. 9, Blk. C-36, Public School Land Survey
Cotton Petroleum Corp. 2-B Lane, 990 FNL, 2310 FWL, Sec. 13, Blk. 2, MK&T RR Survey
Holly Energy Inc. 1 Amacker, 1980 FNL, 660 FWL, Sec. 87, Blk. D, CCSD & RGNG RR Survey
Mobil Oil Co. 57-1 Pegasus Sprayberry Unit, 1980 FSL, 1980 FWL, Sec. 6, Blk. 40 T-5-S, T&P RR Survey
elevations from contour maps published by Ewing
(1990) and Galley (1958) for Texas and southeastern New Mexico, respectively.
The gravity model of the Permian basin strata
above the top of the Ellenburger formation thus
consists of an array of rectangular prisms, 4 km on
a side, centered on grid points in the elevation grid.
The sides of the cells are vertical and extend
between the top and bottom gridded surfaces. The
edges of the model are latitudes 29°N and 35°N and
longitudes 100°W and 105°W. The model consists
of 19,965 grid cells, with additional cells of the
same size added to pad the model out to 167 km
from the edge of the gridded data. Each of the
19,965 gravity model values was calculated using
all cells having centers within 167 km of each
model point.
The calculated effect of the basin model (Figure
6) shows, as expected, that the gravity field due to
the Permian basin strata has the same shape as the
Ellenburger formation topography (Figure 4). The
gridded values representing the effect of the basin
and the gridded values representing the residual
gravity anomaly map (Figure 2) were designed to
coincide. Thus, these values could be subtracted at
each grid point to produce the stripped or geologically corrected deep gravity anomaly map (Figure 7),
which primarily shows the effects of density contrasts below the top of the Ellenburger formation.
A comparison of the magnetic anomaly map
(Figure 3) with both the residual gravity anomaly
map (Figure 2) and the “deep source” gravity
anomaly map (Figure 7) shows that the magnetic
anomalies correlate better with the anomalies in
the deep source map. Because the magnetic
anomalies are due almost totally to basement features, this correspondence indicates that the process used to achieve gravity anomaly separation
was successful. The gravity low associated with the
Abilene gravity minimum (Figure 7) is still present,
and the f lanking gravity highs show a greater
degree of complexity with the smoothing effects of
the Phanerozoic sediments removed. The Abilene
gravity minimum now appears to extend into the
Delaware basin as part of the gravity low west of
the Central Basin platform. Part of the gravity
anomaly associated with the Central Basin platform
has been removed, along with the gravity effects of
the Delaware, Midland, Val Verde, and Marfa basins
(Figure 7).
MODELING OF GRAVITY PROFILES
To derive a detailed interpretation of the Precambrian geology, we constructed computer models of gravity anomalies along four profiles (Figure
1). This process involved integration of all available
geological and geophysical data, so the models
should be viewed as interpretative cross sections of
the structure of the upper crust. Two of the gravity
models cross the Delaware and Midland basins in a
north-south orientation. The other two models are
oriented east-west across the Central Basin platform and intersect the north-south models. The following constraints were used in the gravity modeling: (1) the strike lengths of the model bodies were
limited to distances that are correct for the true
lengths of the individual bodies indicated by the
associated gravity anomalies; this approximation
is used in 2.5-dimensional gravity modeling to
account for the three-dimensional shape of a geologic body; (2) the background density for gravity
modeling is that of an average metamorphic rock
(2.75 g/cc) (Telford et al., 1984); (3) the Phanerozoic geology is divided into density units as illustrated in Figure 5; (4) the geometry of the density units is constrained by data from petroleum
exploration wells; (5) the gravity models were
required to tie at their intersection points; and
Adams and Keller
419
Figure 6—The gravitational
effect of Permian basin
strata between the
surface and the top of the
Ellenburger formation
based on the
three-dimensional
gravitational model
constructed in this study.
The basin is assumed to
have a bulk density of 2.55
g/cc in a crust that has a
density of 2.75 g/cc. This
map shows the gravity
anomaly the Permian basin
would produce if it were
located on a homogeneous
crust.
(6) the models of the Precambrian geology were kept
as simple as possible but had to satisfy all the information available. Thus, the models are more complex than necessary to satisfy just the gravity data.
Two models were created for each gravity profile. The first model created was a basin model that
used densities and formation depths from
petroleum exploration wells to examine the contribution of the structure of the Permian basin to the
gravity anomalies. If the Precambrian basement is
homogeneous, this model should fit the observed
anomalies well. The second model created was a
Precambrian model that started with the basin
model as a constraint and modeled the remaining
gravity anomalies with basement sources. The gravity models will be discussed in pairs, the north-tosouth–oriented models first, then the east-to-west–
oriented models.
The Midland basin model (MM′) is a 320-kmlong, north-to-south–oriented traverse through the
420
Precambrian Basement Geology
Figure 7—“Deep source”
gravity anomaly map of
the Permian basin region.
This map is the result of
subtracting the gravity
caused by the basin
(Figure 6) from the
residual gravity anomalies
(Figure 2). As a result, the
effects of basin structure
have been removed from
the gravity field. The
remaining gravity
anomalies have sources
located below the top of
the Ellenburger formation.
These deep anomalies have
been sharpened by the
removal of the basin
material. The remaining
anomalies match the
magnetic anomaly map
(Figure 3) better than the
original anomalies (Figure
2). This map suggests that
the Abilene gravity
minimum (AGM) exists on
both sides of the Central
Basin platform.
deepest part of the Midland basin and crosses the
Abilene gravity minimum on the east side of
t h e C e n t ra l B a s i n p l a t fo r m ( Fi g u re 1 ) . T h e
basin structure is constrained by data from 171
petroleum exploration wells (Figure 8). The
basin model fits the data poorly; the only place
it fits is across the Pecos arch. In particular, the
40-mGal gravity low associated with the Abilene
gravity minimum is not accounted for by basin
structure.
In the Precambrian model (Figure 8), a 4- to 15km-thick, 120-km-wide, low-density body was used
to model the Abilene gravity minimum. The linear
gravity low of the Abilene gravity minimum could
suggest a basin, but the borehole information from
wells penetrating the Precambrian basement along
the gravity minimum indicate granitic, granodioritic, and dioritic rocks (Flawn, 1956), suggesting that
a granodiorite batholith underlies the gravity feature. Proprietary seismic reflection data over the
Adams and Keller
M
0
50
100
150
200
250
300
M'
-50
-50
mGals
(A)
-100
-100
Obs
Cal
Abilene gravity minimum
-150
0
50
100
150
200
250
-150
300
-50
-50
mGals
(B)
-100
-100
-150
Midland basin
Permian
(C)
Obs
Cal
Abilene gravity minimum
Pecos arch
Permian
0
0
Penn
Depth (km)
-150
Penn
-10
Granitic intrusion
2.64
-10
Granitic intrusion
2.64
-20
Granitic Mafic intrusion
2.93
Intrusion
2.64
-20
421
Figure 8—Midland basin
model. Calculated gravity
values (Cal) from two
different models are
shown. In (A) the
calculated values are the
result of modeling the
profile with the Phanerozoic
basin structure only (i.e.,
the basement was
considered to be
homogeneous and
assigned a single density).
The basin structure was
modeled using the density
units from Figure 5 and is
shown in shades of gray in
(C). This structure is
strongly constrained by
the data from 171
petroleum exploration
wells but does not produce
a match to the observed
gravity data (Obs). In (B),
the calculated values are
the result of adding the
intrabasement features
shown as bodies filled with
patterns in (C). These
calculated values match the
observed data very well.
The intrabasement features
in (C) are consistent with
the rock types encountered
by wells penetrating the
basement and with
regional gravity and
magnetic anomalies.
Precambrian 2.75
0
50
100
200
150
Distance (km)
Abilene gravity minimum near the gravity model
lack ref lections in the Precambrian basement,
which suggests the presence of an intrusive body.
Part of the Abilene gravity minimum east of the
study area is associated with rocks of the Fisher
metasedimentary belt (Muehlberger et al., 1967),
but the gravity low is much more extensive than
the reported extent of these rocks. Thus, we feel
that a batholith is the most likely interpretation for
the origin of the Abilene minimum. The gravity
low located at 215 km along the profile (Figure 8)
is probably caused by a separate granitic body
located south of the Abilene gravity minimum.
This body is represented by a gravity low on the
deep source gravity anomaly map (Figure 7). A
250
300
mafic body in the upper crust is necessary to produce the gravity high located at 40 km along the
model (Figure 8).
The Delaware basin model (DD′) is 250 km long
and traverses the basin from north to south (Figure
1). This model is located on the west side of the
Central Basin platform and extends from the
Northwestern shelf across the basin to the Diablo
platform. The basin structure of this model is constrained by 111 wells. Again, the basin gravity
model fits the observed gravity poorly (Figure 9)
and shows the need for additional bodies within
the Precambrian basement. Specifically, the model
shows that basin structure alone cannot account
for the gravity low located near the center of the
422
mGals
(A)
Precambrian Basement Geology
D0
50
100
150
200
-100
-150
-150
Obs
Cal
Abilene gravity minimum
(B)
0
50
100
150
200
-200
250
-100
-100
-150
-150
Obs
Cal
Abilene gravity minimum
-200
Northwestern Shelf
-200
Diablo Platform
Delaware basin
Post-Permian
Permian
Permian
0
0
Depth (km)
(C)
D'
-100
-200
mGals
250
Devonian
Penn
-10
Figure 9—Delaware basin model.
Calculated gravity values (Cal)
from two different models are
shown. In (A) the calculated
values are the result of modeling
the profile with the Phanerozoic
basin structure only (i.e., the
basement was considered to be
homogeneous and assigned a
single density). The basin
structure was modeled using the
density units from Figure 5 and is
shown in shades of gray in (C).
The structure is strongly
constrained by data from 111
petroleum exploration wells, but
does not produce a match to the
observed gravity data (Obs). In
(B), the calculated values are the
result of adding the
intrabasement features shown as
bodies filled with patterns in (C).
These calculated values match the
observed data very well. The
intrabasement features in (C) are
consistent with the rock types
encountered by wells penetrating
the basement and with regional
gravity and magnetic anomalies.
Mafic
intrusion
2.85
Mafic intrusion
2.85
Granitic
Intrusion
2.64
-10
Granitic Intrusion
2.64
-20
-20
Precambrian 2.75
0
50
100
150
200
Distance (km)
model (Figure 9) or the gravity highs located under
the Northwestern shelf and the Diablo platform.
The Precambrian model requires a granitic body
4–16 km thick and 120 km wide to fit the gravity
low near the center of the model. This body is similar to the one under the Midland basin, and suggests that the Abilene gravity minimum and the
interpreted batholithic source extend into the
northern Delaware basin. The Diablo platform and
Northwestern shelf are associated with paired gravity and magnetic highs, which indicate the presence of high-density and high-susceptibility mafic
250
rocks. A granitic body near 250 km in the model is
required to tie with the model SS′.
The east-to-west–oriented models (SS′ and NN′ )
extend from the Midland basin across the Central
Basin platform to the Delaware basin (Figure 1).
These models target the basement structure of the
Central Basin platform. The southern gravity
model (SS′, Figure 1) is constrained by 140 wells
(Figure 10). This model shows that the structures
of the Delaware and Midland basins can account
for the gravity anomalies in the basins, but cannot
account for either the 40-mGal gravity high over
Adams and Keller
S
mGals
(A)
150
200
250
300
S'
350
Basin only
-50
-50
-100
-100
Obs
Cal
-150
150
mGals
(B) -50
200
250
300
350
Basin with pC
-50
-100
-100
Obs
Cal
-150
Delaware basin
Post-Permian
Permian
CBP
-150
Midland basin
Nellie
well
Figure 10—South gravity model.
Calculated gravity values (Cal)
from two different models are
shown. In (A) the calculated
values are the result of modeling
the profile with the Phanerozoic
basin structure only (i.e., the
basement was considered to be
homogeneous and assigned a
single density). The basin
structure was modeled using the
density units from Figure 5 and is
shown in shades of gray in (C).
The structure is strongly
constrained by data from 140
petroleum exploration wells, but
does not produce a match to the
observed gravity data (Obs). In
(B), the calculated values are the
result of adding the
intrabasement features shown as
bodies filled with patterns in (C).
These calculated values match the
observed data very well. The
intrabasement features in (C) are
consistent with the rock types
encountered by wells penetrating
the basement and with regional
gravity and magnetic anomalies.
Permian
0
0
Devonian
Penn
(C)
Depth (km)
-150
423
2.85
3.00
Granite
2.64
-10
Layered Mafic
Intrusion
-10
-20
-20
Precambrian 2.75
150
200
250
300
Distance (km)
the Central Basin platform or for a gravity low
associated with the eastern margin of the Diablo
platform (170 km in the model). The missing mass
under the Central Basin platform can be accounted
for by a sill-shaped layered mafic intrusion 2–10
km thick centered slightly east of the center of the
platform (Figure 10). The North American Royalties 1 Nellie well, located at 286 km in the model,
penetrated 4.5 km of layered mafic rocks, thus
350
confirming the presence of a layered mafic intrusion in the Central Basin platform (Figure 10)
(Keller et al., 1989). Extension of this body, which
is probably an intrusive complex and not just one
body, northward from this subcrop can account
for the gravity high under the Central Basin platform, but the drilling data show that the body does
not subcrop except in a few places, including Lea
County, New Mexico. The gravity low located at
424
Precambrian Basement Geology
N
mGals
(A)
0
-50
50
100
-100
-100
Obs
Cal
mGals
0
-50
-100
N'
-50
Basin only
-150
(B)
150
50
Basin with pC
100
-150
150
-50
Figure 11—North gravity model. Calculated gravity values (Cal) from two different models are shown. In (A)
the calculated values are the result of modeling the profile with the Phanerozoic basin structure only (i.e., the
basement was considered to be homogeneous and
assigned a single density). The basin structure was modeled using the density units from Figure 5 and is shown
in shades of gray in (C). The structure is strongly constrained by data from 91 petroleum exploration wells,
but does not produce a match to the observed gravity
data (Obs). In (B) the calculated values are the result of
adding the intrabasement features shown as bodies
filled with patterns in (C). These calculated values
match the observed data very well. The intrabasement
features in (C) are consistent with the rock types
encountered by wells penetrating the basement and
with regional gravity and magnetic anomalies.
km thick centered slightly east of the center of
-100 3–5
the Central Basin platform was added to the
Depth (km)
Precambrian model. The presence of mafic rocks is
suggested from some wells drilled into the PreObs
cambrian basement along the northern Central
Cal
-150 Basin platform and by paired gravity and magnetic
-150
anomalies found in the northern part of the Central
Midland basin
Delaware basin CBP
Basin platform and Artesia-Vacuum arch (Figures 3,
Permian
Permian
7). The mafic intrusion along the eastern margin of
0
(C) Penn0
the model is required for the model to tie with
Penn
model MM′.
The northern and southern models show the
necessity of adding mass in the form of mafic bod2.85
to the basement under the Central Basin plat-10 ies
-10 3.00 Layered mafic
form.
The presence of mafic rocks is supported by
Mafic intrusion
intrusion
borehole information and analysis of gravity and
2.93
magnetic anomalies. The two models are similar
in that the mafic body in both models is sill-like.
When all four models are compared, it is apparent
-20 that changes in the basin structure commonly
-20
ref lect changes in the Precambrian basement
Precambrian 2.75
geology, implying basement inf luence on the
Phanerozoic geology of the Permian basin. All
50
the models, when taken together, testify to the
0
100
150
heterogeneity of the Precambrian basement in the
Distance (km)
Permian basin.
170 km in the model can be accounted for by a
granitic intrusion.
The northern gravity model (NN′) is 150 km
long (Figure 1), and its basin structure is constrained by formation depths from 91 wells (Figure
11). As with model SS′, the basin model shows that
the structure of the Permian basin alone, again,
cannot account for the observed anomalies. The
difference between the observed gravity and the
calculated gravity anomalies indicates a need for
additional mass in the profile. To account for the
missing mass, a sill-shaped layered mafic intrusion
DISCUSSION
Classification of Inferred Rock Bodies
The combination of the deep source gravity
anomaly map (Figure 7), the residual magnetic
anomaly map (Figure 3), the results of gravity modeling (Figures 8–11), and the rock type information
from wells drilled into Precambrian basement
allows us to interpret the upper crustal geology of
the region. Our basement geology interpretation is
based on the assumption that the upper crust has a
Adams and Keller
425
Figure 12—Geologic
interpretation of the upper
crustal geology of the
Permian basin region
based on gravity modeling
and comparison of gravity
and magnetic anomalies
with information from
Precambrian outcrops
and wells drilled to
Precambrian basement.
The locations of
Precambrian outcrops at
Van Horn (VH), Pump
Station Hills (PS), Hueco
Mountains (HM), Pajarito
Mountain (PM), Pedernal
Hills (PH), Roosevelt uplift
(RU), and west platform
fault (WPF) are shown.
Interpreted geologic
bodies associated with the
pre-Grenville rift (PGR),
Delaware aulacogen (DA),
Abilene gravity minimum
(AGM), and Crosbyton
anomaly (C) are shown,
along with two proposed
new locations for the
Grenville front in Texas.
The symbols for the well
data are defined in Figure 1.
In the legend, the numbers
in parentheses refer to
families of anomalies
discussed in the text.
density and susceptibility of average metamorphic
rock except where closed geophysical anomalies
are present. A second assumption is that paired
gravity and magnetic anomalies have the same
source body. Under these assumptions, pairs of
gravity and magnetic anomalies may be grouped
into three families, depending on their relative
amplitudes (Figure 12).
The first family consists of pairs of gravity and
magnetic maxima, and several areas of the Permian
basin region contain such paired maxima. These
areas include the Central Basin platform, the
Artesia–Vacuum arch, the Roosevelt uplift, and the
area around Pajarito Mountain in the Sacramento
Mountains of New Mexico (Figure 12). Precambrian basement samples from wells located
426
Precambrian Basement Geology
within these anomalies are dominantly mafic rocks;
hence, we interpret these anomalies as indicating
mafic rocks within the upper crust. This interpretation suggests that a significant volume of mafic
rocks has been intruded into the upper crust of the
region (Figure 12). Age determinations from the
North American Royalties 1 Nellie well drilled on
the Central Basin platform indicate that at least
some of the intrusions are Middle Proterozoic in
age (Keller et al., 1989). Other Middle Proterozoic
intrusions of this age are located in the Roosevelt
uplift and in the area of the Crosbyton geophysical
anomaly (Figure 12) (Adams and Keller, 1994).
The second family of anomalies consists of
paired gravity and magnetic minima. Areas having
this signature include parts of the Abilene gravity
minimum and the Midland basin area. Felsic rocks
are commonly found where this type of anomaly
has been drilled, which leads us to interpret these
pairs of anomalies as indicating granitic bodies in
the upper crust. The largest of these bodies is represented by the Abilene gravity minimum. Rock
samples from wells drilled into this body are granodioritic in composition. Early Paleozoic (preEllenburger) or Late Proterozoic basins are an alternative interpretation for the source of at least part
of some of these anomalies (Figure 12). Pratt et al.
(1992) discuss the possible existence of Proterozoic basins below a veneer of Granite-Rhyolite
terrane rocks in Texas, Oklahoma, and Illinois, and
Keller and Baldridge (1995) show that the Hardeman
basin area contains rocks that are both low density
and reflective beneath the Ellenburger formation.
Thus, there are large areas associated with paired
gravity and magnetic lows and reflective basement
suggestive of sedimentary rocks that are worthy of
further consideration from a petroleum exploration
perspective.
The third family of anomalies consists of gravity
lows paired with magnetic highs. These anomalies
indicate the presence of rocks that have low density and high magnetic susceptibility. These anomalies, where they have been drilled, are commonly
associated with granitic rocks. Some examples of
this family are gravity lows paired with very large
magnetic lows. These anomalies indicate a strong
reversed remanent magnetization formed during
cooling of a granitic body. Areas having this signature include the area north of the Matador arch and
the area of the Roosevelt uplift (Figure 12). These
areas are located within the limits of the Swisher
basement terrane (Figure 1; Denison et al., 1984),
which shows that the mafic rocks in this area are
thin and confirm Flawn’s (1956) interpretation.
These anomalies appear to be related to the southern Granite-Rhyolite province.
Several structures in the Permian basin appear to
be related to the composition of the underlying
basement. The basement highs of the Central Basin
platform, Artesia-Vacuum arch, Roosevelt uplift,
and Pecos arch are associated with mafic upper
crustal rocks (e.g., compare Figures 4, 12). The
deepest parts of the Delaware and Midland basins
are over granitic basement rocks or even Proterozoic basins.
Regional Interpretation of Upper Crustal
Geology
We have formulated a new scenario for the tectonic evolution of the crust in the Permian basin
region (Figure 13) based on work presented here
and results compiled from Tweto (1983), Nelson
and DePaolo (1985), Bennett and DePaolo (1987),
Patchett and Ruiz (1989), Walker (1992), Adams et
al. (1993), Bickford and Anderson (1993), Reed
(1993), Roths (1993), Van Schmus et al. (1993a),
Adams and Keller (1994), Nyman et al. (1994), and
Pittenger et al. (1994). Deformation and accretion
of the outer tectonic belt was the first tectonic
event for which we have a record in this area. This
event produced a continental margin in Texas and
New Mexico along which later tectonic events
occurred. The next known event was development
of the southern Granite-Rhyolite province (Figure
13) and probable formation of the geologic body
associated with the Abilene gravity minimum.
The largest geophysical feature in the area is
the Abilene gravity minimum; it is enigmatic
and has previously been interpreted as marking the location of the Grenville front in Texas
(e.g., Muehlberger, 1965; Denison et al., 1984;
Mosher, 1993). We propose another interpretation
of the Abilene gravity minimum by drawing an analogy with the Sierra Nevada batholith in California
and Nevada. The two bodies have several common
characteristics: (1) both bodies are associated with
approximately 600-km-long gravity lows that parallel present or former continental margins (e.g.,
Condie, 1982); (2) both features are bounded on
one side by small gravity highs associated with
mafic igneous rocks (west side of the Sierra Nevada
batholith and north side of the Abilene gravity minimum); (3) the Sierra Nevada batholith has been
gravity modeled as a 10- to 15-km-thick granitic
body (Oliver, 1977), and the Abilene gravity minimum is modeled as a 4- to 16-km-thick granitic
body here. Therefore, we conclude that the Abilene
minimum probably represents a Middle Proterozoic
continental margin arc batholith similar in origin to
the Sierra Nevada batholith.
Although rocks causing the Abilene gravity minimum have not been dated directly, two possible
ages for its formation are reasonable. The first possible age is 1.34–1.41 Ga, which corresponds to the
Adams and Keller
OUTER TECTONIC BELT
1.7 –+1.6 Ga
++
++++
++++++
NLD?
++
++
1
SOUTHERN GRANITE-RHYOLITE
PROVINCE 1.40 – 1.34 Ga
2
1.7
SGRP
INTRUSIVE
ROCKS
2.0 1.9
1.7 – 1.3 Ga Sm/Nd
Transition Zone
1.3
SGRP INTRUSIVE
AND EXTRUSIVE
ROCKS
AGM
ICM
CONTINENTAL
ARC BATHOLITH
1.4
1.3 Sm/Nd
model ages in Ga
1.4
1.3
Inferred
subduction
zone
PRE-GRENVILLE EXTENSION &
CONTINENTAL MARGIN
> 1.33 – < 1.26 Ga
3
4
GRENVILLE OROGENY &
DELAWARE AULACOGEN
1.3 – 1.0 Ga
PPB
MCR
LA
MCR
RU
DA
PM
MB
CM
EXTENSIONRELATED
MARGIN
1260+20
--
CG 1327+28
--
A
L
5
CBP
H P
F
C
V
CBP
AGDF
DA
EOCAMBRIAN RIFTING
550 Ma
SOA
RIFTED
MARGIN
age of formation of the southern Granite-Rhyolite
province. This age would allow the Abilene minimum to be a source for some of the rocks in the
southern Granite-Rhyolite province (Figure 13).
This age is also consistent with the ages of formation, obtained from U/Pb zircon dating of the
igneous protoliths of metamorphic rocks in the
Llano uplift and Van Horn regions of Texas. A
southern Granite-Rhyolite province age would be
L
PGDF
427
Figure 13—Summary of the
tectonic history of the region.
This figure shows five stages in
the evolution of the Proterozoic
crust of Texas, New Mexico, and
Oklahoma. (1) Formation of the
outer tectonic belt between 1.70
and 1.60 Ga; inferred 1.50–1.60 Ga
continental margin (ICM),
northern limit of 1650 Ma
deformation (NLD?). Sm/Nd =
samarium/neodymium. (2)
Formation of the southern
Granite-Rhyolite province
between 1.40 and 1.34 Ga in a
subduction environment along
with a continental margin arc
batholith represented by the
Abilene gravity minimum.
Southern Granite-Rhyolite
province (SGRP); Abilene gravity
minimum (AGM). (3) Formation
of a pre-Grenville rift and
continental margin (~1.35–1.23
Ga) along an east-west orientation
near the present-day Van Horn
and Llano uplifts. Pajarito
Mountain (PM); Mundy Breccia
(MB); Castner marble (CM); Llano
uplift (L); Carrizo group (CG);
paired gravity and magnetic
anomalies discussed in the text
(A). (4) Grenville orogeny and
formation of the Delaware and
Mid-Continent rift systems
between 1.23 and 1.07 Ga. Pikes
Peak batholith (PPB); Las Animas
basin (LA); preferred Grenville
deformation front (PGDF);
alternative Grenville deformation
front (AGDF); Mid-Continent rift
(MCR); Crosbyton anomaly (C);
Llano uplift (L); Central Basin
platform (CBP); Pump Station
hills (P); Hueco Mountains (H);
Franklin Mountains (F); Delaware
aulacogen (DA); Roosevelt uplift
(RU). (5) Eocambrian rifting and
passive ocean margin formation
(~600 Ma); southern Oklahoma
aulacogen (SOA).
consistent with the apparent crosscutting relationship between the Abilene gravity minimum and the
approximately 1.1-Ga rocks associated with the
Central Basin platform gravity high (Figure 7).
Additionally, Nelson (1990) has interpreted a part
of the Granite-Rhyolite terrane in the St. Francois
Mountains, Missouri, as having been formed by a
subduction-related process, and Patchett and Ruiz
(1989) have suggested that the same process may
428
Precambrian Basement Geology
have produced the Granite-Rhyolite terrane in
Texas. In addition, Nelson and DePaolo (1985) suggest that accretion of new mantle-derived crust,
related to formation of Llano province crust in
Texas, was occurring along the southeastern margin of the continent as early as 1.45–1.40 Ga
(Figure 13). The presence of syn-magmatic deformation of 1.4-Ga plutons in Arizona, Colorado, and
New Mexico is consistent with north-to-northwest–directed regional compression, possibly
caused by a subduction and transpression related
to a plate boundary located to the south or southeast of the intrusions (Nyman et al., 1994). The
location of the Abilene gravity minimum and its
possible origin as a continental margin arc
batholith are consistent with these observations.
Alternatively, the Abilene gravity minimum could
be a granitic batholith related to the Grenville
orogeny. Granitic intrusions associated with
Grenville deformation are found in the Llano uplift
as small posttectonic bodies (Mosher, 1993); however, igneous rocks of this age have not been identified near the Abilene gravity minimum. Structural
vergence of the rocks in the Llano uplift suggests
that subduction during the Grenville orogeny was
directed to the south, but additional information is
needed to be sure of the direction (Mosher, 1993).
Hence, the Abilene gravity minimum batholith is
unlikely to be related to the Grenville deformation,
and circumstantial evidence points toward the
Abilene gravity minimum batholith being related to
the southern Granite-Rhyolite province.
A period of rifting and passive continental margin
development followed formation of the southern
Granite-Rhyolite province. The deformed Middle
Proterozoic rocks that crop out as the Carrizo group
in the Van Horn area (Figure 12) have previously
been interpreted as being related to rifting of the
continent during opening of a pre-Grenville ocean
(Roths, 1993) (Figure 13). The gneisses and schists
in the Llano uplift have been interpreted by
Garrison (1981) and Roback (1994) as being related
to a pre-Grenville arc-trench environment dating
from this time. In the western part of the area, a set
of east-west–oriented paired gravity and magnetic
maxima start near the Van Horn uplift and cross the
southern Delaware basin (PGR, Figure 12). The
trend of these anomalies is consistent with the
trend of the Carrizo group (Figure 1) given by
Denison et al. (1984) and could represent an extension of the Carrizo group in the subsurface. This
trend may also continue east of the Central Basin
platform through Schleicher County (Figures 12,
13). These anomalies may be a part of the rift or
back-arc basin, which formed the pre-Grenville continental margin (Figure 13) near Van Horn. The period of rifting or back-arc spreading and passive margin development may extend from before 1327 ±28
Ma (U/Pb) (Roths, 1993) recorded in the Carrizo
group to after 1260 ±20 Ma (U/Pb) (Pittenger et al.,
1994) recorded in the Castner marble.
The next tectonic event in west Texas was the
complex series of deformations of the rocks of the
Llano and Van Horn uplifts related to the Grenville
orogeny. The deformed rocks in the Llano uplift
mark the minimum northern extent of known
Grenville-age deformation in central Texas (Mosher,
1993). The Van Horn uplift in west Texas marks the
northernmost extent of Grenville-age deformation
in west Texas (Soegaard and Callahan, 1994).
Hence, any boundary of the Grenville deformation
in Texas must pass through the Van Horn area
along the Streeruwitz thrust and must pass north of
the outcrop of Precambrian rocks in the Llano
uplift (Figure 13). The traditional location for the
Grenville deformation front in Texas has been
along the center of the Abilene gravity minimum
(Figure 1). Because we prefer to interpret the
Abilene gravity minimum to be an older granodioritic batholith unrelated to Grenville-age deformation (Figures 12, 13), we need to suggest a more
southerly location for the Grenville deformation
front. Two possible locations for this boundary are
proposed (Figure 12). The first location starts at
the Van Horn uplift and runs east-northeast along
the strong linear gravity gradient at the southern
margin of the Abilene gravity minimum (Figures
7, 12). This boundary is supported by the locations of Paleozoic faults in the Delaware basin
(Ewing, 1990), which may represent a Paleozoic
reactivation and continuation of the Proterozoic
Streeruwitz thrust in the subsurface (Figure 12).
The second possible location is a more southern
linear gravity gradient that is located just north of
the Llano uplift and trends westward toward the
Van Horn uplift (Figures 7, 12). This gravity gradient runs along the northern edge of the Pecos arch
in the Midland basin. At present we prefer the
southern boundary (Figure 13), but additional
study is necessary to confirm this interpretation.
The final feature we discuss is the Delaware
aulacogen (Figure 13). The Permian basin has
been interpreted by analogy with the southern
Oklahoma aulacogen to be the site of an Eocambrian rift (Walper, 1977; Shurbet and Cebull, 1989)
that was structurally inverted during Pennsylvanian–Permian deformation to form the Central
Basin platform, which is analogous to the Wichita–
Amarillo uplift. Drilling the North American
Royalties 1 Nellie well revealed 1.07–1.16 Ga
(U/Pb) age (Keller et al., 1989) mafic rocks, making
the proposed rifting Middle Proterozoic instead of
Eocambrian in age. However, the aulacogen model
can still be considered valid because the late
Paleozoic deformation still reactivates a rift. The rift
is simply older than originally thought. This
Adams and Keller
hypothesis has the advantage of being consistent
with a Tobosa basin containing only a fraction of
the early Paleozoic sedimentary rocks found in the
Oklahoma basin (e.g., Frenzel et al., 1988; Perry,
1989).
The Delaware aulacogen includes the entire
Central Basin platform and the Roosevelt uplift.
The ages of the Delaware aulacogen and Grenville
tectonic event in Texas overlap, indicating a possible relationship between the orogeny and rifting.
The orientation of the Delaware aulacogen is
approximately perpendicular to the orientation of
the Grenville front, possibly indicating a common
stress field between the events (Figures 12, 13).
Extension appears to have been east-west oriented,
which is at a high angle to the northeast (Llano)
and northwest (Van Horn) orientation of compression during Grenville-age deformation in the
region. The collision of continents that have irregular margins or a continent/micro-continent collision could produce a rift at a high angle to the continental margin. The upper Rhine graben may be an
example of this type of rifting (Sengör et al., 1978).
Other areas also contain igneous rocks produced at
this time, including the Crosbyton geophysical
anomaly (Adams and Keller, 1994), the Franklin
Mountains (Roths, 1993), the Town Mountain
Granites of the Llano uplift (Walker, 1992), and
mafic rocks in the Swisher–Debaca basement terrane (Denison et al., 1984). The rifting event is
coincident with formation of the Mid-Continent rift
(Cannon, 1994) and intrusion of diabase sills in
California, Nevada, and Arizona (Howard, 1991).
Thus, west Texas and eastern New Mexico appear
to have been a part of a large igneous province
encompassing most of North America in the
Middle Proterozoic and resulting, at least in part,
from extension.
A period of Eocambrian rifting and continental
development served as the closing tectonic event
of the Proterozoic history of the region (Figure 13).
This event created the continental margin that was
ultimately the site of the Ouachita orogeny. With
the termination of this activity in the early
Paleozoic, the structural framework of the Permian
basin seems to have largely formed. The vast majority of wells drilled in the basin only provide information on younger events, leaving the potential for
many discoveries in the future.
CONCLUSIONS
The Precambrian geologic histor y of the
Permian basin region involves five or six events:
formation of the outer tectonic belt (1.7–1.6 Ga),
southern Granite-Rhyolite province volcanism
(1.40–1.34 Ga), and pre-Grenville extension
429
(1.33–1.23 Ga); Grenville-age deformation (~1.3–
1.0 Ga); formation of the Delaware aulacogen
(1.16–1.07 Ga); and Eocambrian rifting and continental margin formation (Figure 13).
The Abilene gravity minimum is the most prominent anomaly in the region, and we interpret it to
be due to a granitic batholith, which is comparable
in size to the Sierra-Nevada batholith. Several lines
of evidence suggest this batholith is related to formation of the southern Granite-Rhyolite province,
which pre-dates Grenville deformation, so we have
proposed two new, more southerly locations for
the Grenville front (Figures 12, 13).
The Central Basin platform is associated with a
gravity high that is caused by a layered mafic intrusion. This feature is associated with a Middle
Proterozoic (~1.1 Ga) rift, the Delaware aulacogen,
and numerous other mafic bodies of this age are
found in the region. This age coincides with the
formation of the Mid-Continent rift system and is
syntectonic to posttectonic to Grenville-age deformation in Texas (Figure 13).
Many areas in the Permian basin are dominated
by low-density and low-susceptibility features that
could be granitic intrusions or the sites of preEllenburger basins. These basins may contain
unmetamorphosed sedimentary rocks similar to
the Allamoore group, Hazel Formation, or Van
Horn Formation, sometimes below a thin layer of
southern Granite-Rhyolite province rocks.
Precambrian basement structure and geology
exerted significant control on Phanerozoic basin
structure and sedimentation patter ns in the
Permian basin. Reactivation of Precambrian basement structures in the Paleozoic played a role in
development of Paleozoic basin structures and the
pattern of sedimentation in the Permian basin, both
of which provide control on the distribution of
source rocks and reservoirs in the basin. The uplifts
of the Permian basin are associated with mafic
rocks in the Precambrian basement, and the basin
deeps are associated with felsic rocks and possible
Proterozoic basins.
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ABOUT THE AUTHORS
Donald C. Adams
G. Randy Keller
Donald Adams received his B.S. degree in
applied geophysics and M.S. degree in geophysics
from the Michigan Technological University. He
received his Ph.D. in 1995 from the University of
Texas at El Paso. He is currently a senior geophysicist with the Exxon Exploration Company.
Randy Keller is chairman and L.A. Nelson
Professor in the Department of Geological Sciences
of the University of Texas at El Paso (UTEP). He
came to UTEP in 1976 from the University of
Kentucky and received his Ph.D. concentrating in
geophysics from Texas Tech University in 1973. His
research interests include seismology, gravity, magnetics, tectonics, and integrated geophysics, and he
has published over 100 articles on these subjects.
He is an associate editor of Geophysics and the
Geological Society of America Bulletin.