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Salt Tectonics, Associated Processes, and Exploration Potential: Revisited:
1989–2019
67
Middle Jurassic Tectonic Models for the Gulf of Mexico in Light of New
Bajocian Ages for Proximal Margin Salt Deposition
James Pindell
Tectonic Analysis Ltd. and ION E&P Advisors
Chestnut House, Burton Park
Duncton, West Sussex GU28 0LH UK
email: jim@tectonicanalysis.com
Diego Villagómez
Tectonic Analysis Ltd. and University of Geneva
Rue des Maraichers 13
CH-1205 Geneva, Switzerland
Brian W. Horn
ION E&P Advisors
2501 City West Boulevard
Houston, Texas 77042, USA
Roberto Molina Garza
Centro de Geociencias
Universidad Nacional Autónoma de México
Campus Juriquilla, Querétaro, México
Abstract
Recent Bajocian Sr87/86 ages (169 Ma) for
Louann, Campeche, and other evaporite samples in the
northern, southern, and western proximal rims of the
Gulf of Mexico Basin (GoM) suggest that new perspec-
tives on the basin’s rift and drift history are warranted.
Presently, sea-floor spreading is believed to have
started in the Oxfordian because (1) salt is usually con-
sidered Callovian-early Oxfordian and the basinward
limits of Louann and Campeche salt, having been sepa-
rated by spreading, closely match the limits of the
intervening oceanic crust; and (2) Yucatán and the
reconstructed, little-faulted, base-salt unconformity
along GoM margins can be accommodated neatly into
current early Oxfordian (~160 Ma) North America-
South America tectonic reconstructions. However,
reconstructions for Bajocian time (~169 Ma) generally
do not accommodate the great area of this nearly planar
unconformity. Three possible explanations are evalu-
ated here: (1) that Bajocian North America-South
America reconstructions are too tight; (2) that salt
deposition began in the Bajocian but persisted through
Bathonian–early Oxfordian time before spreading and
hence offlapped from the dated locations along the
proximal GoM margins; and (3) that all salt is Bajocian,
but that it flowed basinward to the eventual line of ini-
tial Oxfordian spreading. We assess explanation 1 by a
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sensitivity analysis of the Equatorial Atlantic compo-
nent of the circum-Atlantic reconstruction, concluding
this option is viable. If correct, explanation 1 implies a
Bathonian rather than an Oxfordian onset of seafloor
spreading, along with several other important aspects of
GoM evolution. Explanations 2 and 3 imply ongoing
Bathonian-early Oxfordian basement expansion which
is not readily identifiable in seismic data, but which
might be allowable if the base-salt surface beneath the
Sigsbee slope is eventually shown to be non-planar and
if salt deposition is shown to be younger in the eastern
GoM than in the western/central GoM. These options
will thus remain viable unless Sr isotope ages for distal
salt eventually prove to be Bajocian, as in the proximal
margins. We find explanation 3 least likely because lit-
toral-neritic paleo-environmental assignments for
Upper Jurassic strata in certain offshore wells suggest
maintenance of shallow water conditions above salt far
into the offshore and hence only minor salt deflation by
that time. The new ages suggest that the Huayacocotla
area of central, continental Mexico is the most likely
link to the world ocean in Bajocian time because Bajo-
cian-Bathonian Huehuetepec and equivalent evaporites
occur there. Further, we highlight and suggest alterna-
tives for resolving a long-standing conundrum posed by
the thin salt package lying upon the little-faulted,
postrift, base-salt unconformity in the eastern GoM that
lies at the same structural level as the oceanic crust. We
suggest that deep-water salt deposition may provide a
viable alternative explanation to outer marginal col-
lapse at the rift-drift transition. Finally, we present a
195 Ma (Early Jurassic) reconstruction using the looser
Equatorial Atlantic fit which shows that the looser fit
accords well with reconstructions of western Pangea.
Introduction and Background
Numerous strontium isotope age determinations
indicate that the Louann and Campeche evaporites in
the Gulf of Mexico (GoM) are Bajocian in age. This
paper addresses the impact these ages have on models
of the tectonic evolution and certain stratigraphic rela-
tionships for the GoM.
Figure 1 shows features around the Gulf of Mex-
ico (GoM) pertinent to this discussion. Concerning the
limit of the oceanic crust, the solid green line segments
are constrained by direct observation in ION seismic
lines, whereas the dashed green segments are less cer-
tain due to lack of data or to poor imaging of basement
through complex salt structure. This uncertainty contin-
ues to hinder precise reconstructions of the conjugate
limits of oceanic crust, but because exhumation of sub-
continental mantle at the rift-drift transition serves as an
alternative mode of plate separation (in addition to sea-
floor spreading), such precise reconstructions are
perhaps less feasible than once envisioned. In general,
we find in the examination of seismic reflection data
that the “tie-point” in the wishbone geometry formed
by (1) the top of the oceanic crust, (2) the basinward-
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dipping base-salt unconformity of the outer continental
margin, and (3) the landward-dipping outer marginal
detachment or basement step-up beneath the distal mar-
gin, is usually imaged. The characteristic geometry of
this tie point serves as a good proxy to the limit of the
oceanic crust (for examples, see Pindell et al., 2011;
2014 for the Florida margin, and Horn et al., 2017 for
the Yucatán margin).
The purple salt basin limits in Figure 1 are modi-
fied after Salvador (1987; 1991) from observation of
ION and Pemex seismic data. Note the close correspon-
dence in the northeast GoM of the onlap limits of the
salt and the Upper Jurassic marine formations (e.g.,
Smackover Formation; Dobson and Buffler, 1997).
This suggests that sea water had filled the entire rift
basin, such as it was at that time, to global sea level
during salt deposition. This “filled to the brim” argu-
ment is also supported by the littoral to neritic (i.e.,
<200 m water depth) paleo-environmental assignments
for the Upper Jurassic marine section at the offshore
Norton (Fig. 1) and several other offshore wells that
reached Jurassic strata (BOEM data base, 2019; Pindell
and Kennan, 2007; various personal communications).
Although these wells have been drilled on mobilized
salt-related structural highs, the Late Jurassic deposi-
tional surface as a whole was apparently quite near to
global sea level (within ~500 m) along much of an off-
shore shelf that was made shallow by thick salt beneath.
Based on stratigraphic juxtaposition, Salvador
(1987; 1991) argued that the age of the Louann and
Campeche salt units are Middle Jurassic, probably
Callovian as suggested by the marine strata of the
Tepexic Formation of the Huayacocotla Embayment in
east-central Mexico (Fig. 1), which is very close to the
GoM. Salvador also argued that the Huayacocotla area
of Mexico served as the shallow marine connection to
the world ocean (breach of the basin rim). Here, we
point out that a continental evaporite unit, the Huehuet-
epec Formation, underlies the Callovian Tepexic
Formation having a presumed Bathonian–Callovian age
(Rueda-Gaxiola, 2009). In addition, there are several
Bajocian marine sections across the Oaxaca region of
southern Mexico (Sandoval and Westermann, 1986;
Fig. 1), completing a possible path for Bajocian sea
water from the Pacific to the GoM. Thus, we know that
Pacific marine waters encroached to within a couple
hundred kilometers of the nascent Gulf of Mexico rift
basin during Bajocian time. Furthermore, as far as we
know, there was no clear continuous basement high,
apart from the relatively small Tuxpan block (Fig. 1),
between the Huayacocotla and the nascent GoM basins.
Presently, we agree with Salvador that this was the most
likely, documentable source of water for GoM salt, but
we emphasize that GoM salt deposition began and was
possibly completed in the Bajocian.
Both Buffler and Sawyer (1985) and Pindell
(1985) noted the close correspondence between the
majority of the basinward limit of salt and the approxi-
mate limit of the oceanic crust mapped at that time.
This relationship has been instrumental in the long-
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standing belief that sea-floor spreading in the GoM
began in the Oxfordian, just after the supposed Callo-
vian salt deposition, such that the salt was dissected into
its two halves (Louann and Campeche) by rotational
sea-floor spreading (Humphris, 1978; Pindell and
Dewey, 1982).
More recently, modern seismic reflection data
show that the GoM salt is essentially a postrift but pre-
drift depositional unit (Diegel et al., 1995; Peel et al.,
1995; Pindell et al., 2011; 2014; 2015; Rowan, 2014).
This is based on the observation that salt generally rests
on a little-faulted, top-rift or base-salt unconformity
except for within about 10-20 km of the limit of the
oceanic crust. It is therefore commonly believed that
the map-view area of this unconformity, when Yucatán
is reconstructed back to the conjugate U.S. margin, had
already formed between these margins at the time of
salt deposition, as allowed by the paleo-positions of
North and South America and the rotation of Yucatán.
Salt sample positions 1-5 in Figure 1 have pro-
vided Sr isotope ratio ages of 167.5-170 Ma, essentially
Bajocian. These include dates from Land et al. (1988);
Pulham (2016, personal communication); Snedden et
al. (2018); and Pindell et al. (in press 2019). Several of
these determinations are on samples from the interior
salt basins, but Pindell et al. (in press 2019) show 169
Ma ages (Bajocian) for both the main Louann salt body
(Hockley salt mine near Houston) and the Campeche
Salt (Bacab-21 well, just north of the Cantarell Field).
Pindell et al. (in press 2019) also show 166 Ma (anhy-
drite) and 169 Ma (halite) ages for the salt penetrated in
the Mata Espino 101-B well of the Veracruz Basin,
Mexico, but this salt is judged in that paper to be
derived from the continental platform beneath the
Sierra Madre Oriental thrust belt. If so, this salt is likely
correlative with the nearby Huehuetepec salt (see
above) of the Huayacocotla embayment. It also proba-
bly has assisted with the décollement surface for the
Sierra Madre, along with Upper Jurassic shales (R. Gra-
ham, personal communication, 2017).
Numerous, similar postrift, predrift reconstruc-
tions of the Yucatán and southern U.S. margins have
been made by rotating Yucatán clockwise back to the
approximate U.S. continental limit, purporting to
remove the oceanic crust in the Gulf of Mexico (Pindell
and Dewey, 1982; Pindell, 1985; Molina-Garza et al.,
1992; Schouten and Klitgord, 1994; Pindell and Ken-
nan, 2001; 2009; Imbert and Philippe, 2005; Hudec et
al., 2013; Nyugen and Mann, 2016; Pindell et al., 2016;
Lin et al., 2019). The commonly accepted age for such
reconstructions is Oxfordian due to the salt–oceanic
crust relationship mentioned earlier and the presumed
age of Callovian-early Oxfordian for salt. In most of
these reconstructions, the estimated Oxfordian separa-
tion between North and South America is just large
enough to accommodate Yucatán in an orientation that
allows inclusion of the base-salt unconformity between
Yucatán and the US. However, if the salt along the
proximal margins of the GoM is Bajocian, we must
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seek explanations for the apparent space problem that
this creates.
Three possible explanations are evaluated in the
sections below: (1) that Bajocian North America–South
America reconstructions are too tight; (2) that salt
deposition began in the Bajocian but persisted for about
8 m.y. through the Callovian-early Oxfordian, offlap-
ping from the dated locations in the proximal GoM
margins and encroaching into the eastern GoM; (3) that
all salt is Bajocian, but that the salt flowed basinward to
the limit of oceanic crust during Bathonian-early
Oxfordian crustal extension/creation.
Sensitivity of the Equatorial Atlantic Fit, and Impact on North America–South America
Plate Kinematics
We will start by assessing the first of the three
above explanations. The relative motion framework
between the Americas is defined by reconstructing the
three-plate system North America-northwest Africa-
northeastern South America (Ladd, 1976; Pindell,
1985). Fair consensus has been reached for the early
opening history of the Central Atlantic (compare the
opening histories in Labails, et al., 2010; Kneller et al.,
2012; and Pindell, 2014). Concerning Africa and South
America, we need only to concern ourselves with the
Equatorial Atlantic full closure fit because the GoM
opened prior to the opening of the Equatorial Atlantic.
Much of the crustal structure of the Equatorial Atlantic
margins has been strongly obscured by complex defor-
mation and voluminous sedimentation during both syn-
rift and Cenozoic (Amazon Shelf and Niger Delta)
times; consequently, a critical look at this reconstruc-
tion with modern data sets is warranted.
Figure 2 allows comparison of several Equatorial
Atlantic reconstructions by showing the present-day
coastline positions for each, which are based on various
data sets including public seismic reflection records and
the fracture zone traces defined by satellite gravity
images. The reconstructions are similar, but differences
of 100-150 km along the margin indicate room for
refinement. Recently, we have begun to assess the pros
and cons of various Equatorial Atlantic reconstructions
by examining seismic data sets, including those of ION
Geophysical. It is a daunting task to reach the desired
level of accuracy and includes assessment of such
parameters as: defining the synrift stretching direction
and early changes therein; estimating the volume of
magmatic addition to the crust during primary rifting
and also during the secondary passage of the en-éche-
lon mid ocean ridge segments; and judging how much
of the margins are true continental crust versus synrift
sediments that were deformed during transpressional
motions between conjugate transform segments.
Although this effort is still underway, we pres-
ently believe that the tighter fits in the east of Figure 2
(approaching Nigeria) are most satisfactory and that the
looser fits in the western reaches of Figure 2 (Liberia)
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are most satisfactory. The heavier black line (Fig. 2)
lies amidst previously published estimates at both ends
of the reconstruction and is our “working ION fit” for
the prerift reconstruction (work is still underway). This
reconstruction appears only marginally different to the
others shown in Figure 2. However, when the revised
reconstruction is projected 700-800 km to western Ven-
ezuela and Colombia, we find that the relationship
between North and South America in the three-plate
loop (South America-Africa-North America) is some
200 km looser than, say, using the Equatorial Atlantic
fit of Pindell et al. (2006). This difference allows the
Yucatán Block to be placed farther from the US margin
in Bathonian reconstructions, thereby avoiding the
space problem for the base-salt unconformity noted
above. Thus, our working ION Equatorial Atlantic fit
(Fig. 2) allows the estimated area of autochthonous salt
deposition, as judged by the reconstruction of the little-
faulted base-salt unconformities from the US and
Yucatán margins, to have formed already in Bajocian
time. Thus, this explanation is a viable but still uncer-
tain solution to the Bajocian space problem.
Concerning the second of the three possible
explanations, above, that salt deposition might have
persisted longer than Bajocian and hence become
younger in offshore areas, with offlapping architecture
through Bathonian-Callovian time, we know of no pub-
lic data with which to assess this possibility. The faunal
and palynological data from Sigsbee Knolls off north-
west Yucatán, for example, do not constrain the age any
more precisely than Bajocian-Kimmeridgian (Kirkland
and Gerhard, 1971). Therefore, we cannot rule this
option out. On the other hand, we may ask a question
that may be pertinent to this option. If sea water reached
the presently mapped onlap limit of salt in the Bajocian,
then why did salt deposition not continue in such proxi-
mal areas in the Bathonian or the Callovian? Two
obvious explanations are that (1) the basin was not
filled completely to global sea level during those times,
or (2) the proximal rim of the GoM basin (north and
south) had been slightly uplifted after initial salt deposi-
tion, possibly due to flexural or thermal elevation in
relation to continued rifting in the basinward direction
(Watts et al., 1982), or to isostatic rebound resulting
from basinward crustal extension (Weissel and Karner,
1989). The lack of polar glaciation during the Middle
Jurassic probably precludes sea level change as another
possible option.
The third possible explanation, above, that all salt
is Bajocian but that it flowed basinward for 8 m.y. until
an Oxfordian onset of seafloor spreading, comes with
the prediction that the upper surface of the salt should
have been deflated to some degree during flow, without
continued salt deposition, such that the average eleva-
tion of this surface ended up significantly below global
sea level. However, the Norton and several other off-
shore wells that tagged Upper Jurassic section indicate
littoral to neritic (<200 m) water depths during that time
(BOEM database, 2019; Pindell et al., 2007), suggest-
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ing only minor deflation and casting doubt on this
option.
Until strontium dating of offshore samples is per-
formed and made public, we consider explanations 1
and 2 as viable. However, each of these raises other
important questions that are considered in the next
section.
Building Bajocian Salt into the GoM Evolutionary Model
Figures 3 and 4 show Bathonian (167 Ma) recon-
structions using the Equatorial Atlantic fits of Pindell et
al. (2006) and the looser “working ION fit” of Figure 2,
respectively. The depicted area of salt deposition in Fig-
ure 3 necessarily omits much of the area beneath the
Sigsbee slope and between western Florida and north-
ern Yucatán and thus is not as large as the entire
mapped area of the little-faulted base-salt unconfor-
mity. In contrast, Figure 4 does accommodate the entire
mapped area of the unconformity. Figure 3 thus accords
with the idea that salt deposition was younger or
offlapped toward the basin center in Bathonian-early
Oxfordian time as extension continued in the western
and central GoM margins prior to sea-floor spreading.
In addition, the fit of Figure 3 suggests that salt deposi-
tion may have encroached eastward into the rift zone
between western Florida and northern Yucatán after
further stretching and eastward rift propagation in
Bathonian-early Oxfordian time. Unfortunately, none of
the public measured strontium ages to-date (Fig. 1) are
from this eastern GoM rift zone, so this possibility is
untested.
Figure 4 shows South America about 200 km far-
ther south than Figure 3 so that the entire base-salt
unconformity fits between Yucatán and the US margin
at 167 Ma. In fact, the position for Yucatán in Figure 4
is equivalent to the 159 Ma position of Pindell et al.
(2016) relative to North America; thus, this 167 Ma fit
provides the same, but earlier, starting point for sea-
floor spreading in the GoM based on the anomaly pat-
terns from the Pemex aeromagnetic map and several
ION seismic lines that corroborate the limits of oceanic
crust. Therefore, by the same arguments used since the
1980’s to suggest an Oxfordian onset of spreading, Fig-
ure 4 with its looser Equatorial Atlantic fit suggests that
the spreading in the GoM may have begun in the Batho-
nian (167 Ma).
The possibility of a Bathonian onset of spreading
in the GoM raises some interesting implications.
(1) It implies shorter synrift and longer drift
stages than previously thought, and points to relatively
faster synrift and slower sea-floor spreading extension
rates. (2) It suggests that postsalt, deep-water (>2 km)
marine deposition on the oldest parts of the oceanic
crust may have begun in the Bathonian rather than in
the Oxfordian, a question that remains untested.
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(3) It suggests that the Oxfordian transgression of
open marine sea water onto the GoM’s proximal mar-
gins (e.g., Smackover Formation) was delayed by some
mechanism relative to initial marine deposition in the
basin center. Basin flank flexural uplift or thermal infla-
tion (Watts et al., 1982) or remnant topography from
extensional isostatic rebound (Weissel and Karner,
1989) are possible mechanisms for delayed onlap.
(4) The outer portions of the rifted continental
margins and the landward fringes of the oceanic crust
(Fig. 1) would have had an additional 8 m.y. in which to
undergo postrift cooling prior to the deposition of
Oxfordian source rocks (Pepper, 2016) along the mar-
gins. On the other hand, the possible Bathonian-
Callovian section predicted by this model along the dis-
tal parts of these margins may have been source-prone,
as well.
In addition, the new Bajocian salt age carries two
additional implications. First, the time span of the
“Norphlet–Ek Balam depositional window” (coastal
dune, beach, and fluvial clastic sections separating the
mainly continental and open marine deposition in the
northeast GoM and along Campeche Basin) is
expanded in both the Figure 3 and Figure 4 models
(Pindell et al., 2019). Second, the Bathonian and the
Callovian stages can be removed from the period con-
sidered as the synrift/sag phase and added to the period
considered as the drift phase.
A Persistent Problem with Salt Deposition Models
The considerations above allow us to highlight
and examine one of the more outstanding problems in
our understanding of the salt-related evolution of the
Gulf of Mexico, and possibly with rifting in general.
Most workers accept that salt deposition occurs by des-
iccation of shallow restricted marine water. This can
occur along coastal fringes at global sea level or in geo-
graphically isolated air-filled depressions well below
sea level. The latter setting is commonly argued for
very thick accumulations of salt deposited upon planar
basal unconformities because the accommodation space
is created prior to salt deposition; hence, no further con-
tinental rifting is called for. All that is needed is a large
enough number of marine floods and desiccations to fill
most or all of the depression with salt. However, Pindell
et al. (2011; 2014; 2018) have questioned this model
after observing that only a very thin layer of salt was
deposited upon the little faulted, base-salt unconformity
along the outer northwest Florida margin at the same
structural level as the adjacent oceanic crust. This is
true for the northern Yucatán margin, Florida’s conju-
gate, as well (Horn et al., 2017). A thin salt in this
position would be acceptable in the air-filled depression
model if the salt was limited to that deep position; it
could be postulated that the depression was only par-
tially filled with salt. Pindell et al. (2014) noted,
however, that this thin salt layer in the eastern GoM
continues landward, passing around the Florida Middle
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Grounds Arch, to roughly the same onlap limit as the
Upper Jurassic open marine Smackover Formation
(Fig. 1) and, hence, that correlative salt must have been
deposited near global sea level. These authors also
noted that backstripping of the eastern GoM oceanic
crust (water loaded) only returns that crust to about 2.5-
3 km subsea at the time of crustal accretion, pointing to
the “normal” character of the oceanic crust in the GoM
(i.e., about 6-7 km thick). The question, then, is how to
explain a thin salt layer (<200 m thick?) spanning the
entire distance from the global marine strand line at sea
level to the oceanic crust formed at about 2.5 km depth,
if there is no faulting of the base-salt unconformity on
which to blame postsalt tectonic rates of subsidence.
To account for this, Pindell et al. (2014) suggest
the concept of “outer marginal collapse” (Fig. 5A). This
concept employs the only structural feature in distal
margin cross sections that might account for a tectonic
(i.e., rapid) rather than a thermal (slow) subsidence of
the outer margin, namely the subcontinental Moho, or
outer marginal detachment, where it emerges from
beneath the thinned continental margin to meet the oce-
anic crust. The model applies most directly to margins
where continental breakup, as opposed to rifting, is rel-
atively magma poor, such as the GoM, but it also can be
applied to magma-rich margins where the structures
thought to be responsible for outer marginal collapse
are buried in lava (often seaward-dipping reflectors).
Those authors propose that the thinned continental mar-
gins effectively slide off the rising mantle welt
associated with the onset of sea-floor spreading, which
must involve a normal-sense shear zone, the outer mar-
ginal detachment. This shear zone appears to allow for
the emergence/exhumation of the sub-continental man-
tle prior to sea-floor spreading, during which it
becomes serpentinized.
Normal shear along this structure has been pro-
posed by Pindell et al. (2014) as a mechanism for rapid
(tectonic) subsidence of distal continental margins to
the depths of the nascent oceanic crust (~2.5 km) at the
time of the rift-drift transition. We consider that this
shear could help to accommodate another process that
has been proposed to account for rapid subsidence
without upper crustal faulting, namely depth-dependent
stretching of only the lower crust (Driscoll and Karner,
1998; Karner and Gambôa, 2007). If so, then lower
crust as well as mantle may comprise the exhumed
material between the upper continental and the oceanic
crusts.
There are two other possible options for resolving
how a thin, postrift layer of salt might occur at depths
where accretion of typical oceanic crust begins (Figs.
5B and C).
(1) The North Yucatán-Florida rift zone was held
anomalously high by mantle flow during margin devel-
opment (i.e., dynamically supported), such that sea-
floor spreading (of normal thickness oceanic crust)
began near global sea level. Such a setting appears to be
rare, and subsequent subsidence of not only the margin
but also of the early oceanic crust would need to out-
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pace routine rates of thermal subsidence, presumably in
the Late Jurassic, to reach the depths observed today.
This rapid subsidence would also need to have been
dynamically controlled. We consider this scenario pos-
sible, but overly contrived and do not consider it
further.
(2) Salt deposition occurred in deep water, but
never filled the eastern GoM rift basin to sea level, by
precipitation from hypersaturated solution. The feasi-
bility of this possibility stems from recent studies of the
Dead Sea that show salt is precipitating directly from
the hypersaturated water column on the sea floor today
(Anati, 1993), without desiccation of the basin. The
Dead Sea reaches 304 m water depth at roughly 730 m
below sea level. Similarly, salt deposition in the Medi-
terranean is thought to have occurred in remnant
isolated water bodies 1 to 2 km deep (Roveri et al.,
2014), rather than by complete desiccation. Regarding
the air-filled depression hypothesis in which salt is
deposited in preexisting accommodation space above
the continental margins, the thin salt in the deep eastern
GoM may be explained simply by less salt being depos-
ited there (in deep water) relative to the western/central
GoM where salt filled the basin to global sea level as
shown by the salt onlap limits. We will now explore
this possibility further in reference to the alternative
models of Figures 3 and 4.
Consider the depth of the subsalt unconformity
along the distal margins at the time of salt deposition
prior to the onset of seafloor spreading. Disregarding
the possibility of outer marginal collapse, the fact that
the distal unconformity, where observable, lies at simi-
lar structural levels as the pending oceanic crust
suggests that the distal margins were similar to young
oceanic crust in terms of buoyancy. That is, if they were
loaded by water but not by sediment or salt, they should
have been situated about 2.5 km subsea. However, to-
date there is little indication that the basin was filled
with either marine or lacustrine water prior to salt depo-
sition. If the unconformity was subaerial prior to salt
deposition (as we expect), the depth below sea level of
the unconformity prior to salt deposition should have
been on the order of 1.7 km because 2.5 km of water
provides a similar load as about 800 m of mantle.
The paleogeographic scenario provided by Figure
4 implies that the most distal parts of the base-salt
unconformity should have been roughly 1.7 km below
sea level just before marine inundation at 169 Ma, when
the onset of sea-floor spreading was imminent (Batho-
nian). This depth would have been somewhat less if
synrift and sag sediments were significant near the
eventual line of initial spreading. Southern Mexican
stratigraphy (see earlier) and the new salt ages suggest
the existence of a Pacific connection in the Bajocian
and that the basin was filled to global sea level to pro-
duce epicontinental evaporites (see earlier).
Upon flooding, the subsalt depositional surface of
the deep basin center would have deepened geologi-
cally quickly (tens of thousands of years judging from
Pleistocene glacial rebound rates) to about 2.5 km sub-
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sea in the distal margins as sea water loaded the basin.
Salt was then deposited by evaporation cycles and/or
direct precipitation from hypersaturated solution across
the entire area of the reconstructed base-salt unconfor-
mity. The load of the salt as opposed to water should
have caused further deepening of the base-salt uncon-
formity, potentially up to some 4.5-5 km subsea in
distal flanks of the margin if loaded with salt to sea
level. However, we know that salt remained thin in the
eastern GoM margins (western Florida and northern
Yu c at á n ) .
If the basin was filled with sea water to global sea
level to allow for Bajocian salt deposition to reach a
similar onlap limit as the Oxfordian open marine sec-
tion, then an inefficient, deep-water salt depositional
mechanism might be postulated in the paleo-eastern
GoM, thus leaving only a thin salt there. This may have
been due to (1) subregional fluvial input of fresh water
from a Nile-like river with headwaters in a wetter cli-
matic zone, or (2) a more humid climate in the eastern
GoM than that of the western/central GoM, possibly
because of relative proximity to the southern Central
Atlantic Ocean. On the other hand, if the Atlantic was
the source of most of the GoM sea water, with possible
entry points across the South Georgia or South Florida
basins (Fig.1), then perhaps the thinner salt is due to
eastern GoM water being less hypersaturated on aver-
age than western/central GoM water.
Next, we explore how deep-water salt deposition
may have occurred in the Bajocian-Bathonian paleo-
geographic scenario for the GoM implied by Figure 3.
This model is more complex because the area of the
base-salt unconformity is greater than the size of the
nascent GoM basin in the Bajocian, and thus rifting
must have continued during and/or after Bajocian salt
deposition. This model also holds that, in the Bajocian,
the central and western GoM margins accommodated
salt deposition to the maximum onlap limit allowed by
sea level, whereas the eastern GoM margins (Florida
and northern Yucatán) and the area beneath the Sigsbee
slope were still expanding by continental extension or
magmatic accretion. Further, the eastern GoM margins
are presumed to have been above global sea level so as
to escape salt deposition. Beneath the Sigsbee slope,
tectonic extension or magmatic accretion (some form of
sea-floor spreading beneath salt) likely continued pre-
sumably below sea level but seismic data cannot
determine if a planar base-salt unconformity is present
there (Fig. 1, dashed oceanic limit). In this model (Fig.
3), salt deposition will continue through Bathonian-
early Oxfordian time in the central and western GoM
with water levels below the proximal basin rims, lead-
ing to an offlapping salt architecture.
In the model of Figure 3, rifting in the eastern
GoM was still occurring in the Bathonian-early Oxford-
ian. Although the floor of the eastern GoM basin may
have fallen below global sea level by then, it likely
remained above the water level in the central/western
GoM as the base-salt unconformity finished forming
without salt deposition. We do not know the age of salt
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78
deposition in the eastern GoM, but it must have been
delayed until the very latest stages of rifting there,
which would have been in the late Callovian-early
Oxfordian in the scenario of Figure 3, once the basin
had become larger.
By the time the base-salt unconformity had
formed everywhere in the GoM in the Figure 3 sce-
nario, the axis of the Florida-northern Yucatán
subaerial rift zone probably approached 1.7 km depth
below global sea level (see above). This implies that the
water level in the central and western GoM, where salt
deposition continued during Bathonian-early Oxford-
ian, was even lower. However, once sea water advanced
into the eastern portion of the GoM basin, the thinned
continental crust would become water-loaded. When-
ever the basin was filled to global sea level, the distal
base-salt unconformity would have been depressed to
about 2.5 km below global sea level as suggested by the
similar depth of the adjacent oceanic crust, the forma-
tion of which was imminent. The thin salt of the eastern
GoM could have been deposited either at the start of,
during, or at the end of this marine invasion and subsid-
ence of the unconformity. The former would allow for
shallow water salt deposition in the eastern GoM, but
either of the other two would imply deeper water depths
during salt precipitation. The complex model of Figure
3, which predicts diachroneity in both rifting and salt
deposition, implies that the eastern GoM salt was
deposited within the Callovian-early Oxfordian inter-
val, but to our knowledge there are no public evidences
for deposition specifically during this interval in the
GoM basin.
A final consideration concerning deep-water salt
deposition in both the Figure 3 and Figure 4 paleogeo-
graphic models above is the depth of the upper salt
surface at the end of salt deposition. As mentioned ear-
lier, we accept that the littoral-neritic depth calls for the
Upper Jurassic at Norton and other wells are valid, and
thus we believe that salt had filled the western/central
GoM to near sea level (within, say, ~400 m) by early
Oxfordian time. This would imply a salt thickness at
that time approaching 4 km for much of the conjugated
basin, prior to sea-floor spreading. In contrast, the top
of the eastern thin salt was likely 2.5 km depth at that
time. Thus, the upper salt depositional surface likely
had a 2 km bathymetric gradient that deepened from
west to east, while the depth of the base-salt unconfor-
mity increased from east to west due to the greater load
of salt in the west/central area. If this depiction of a
high salt volume and Late Jurassic basin physiography
is roughly correct, then considerable salt dissolution
through time, especially since Late Cretaceous, may be
implied.
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Summary and Further Implications
We have explored two different Middle Jurassic
paleogeographic scenarios for the GoM in light of
recent evidence (strontium isotope dating) that at least
the proximal GoM salt (around the basin rim) was
deposited in the Bajocian. One scenario considers dia-
chronous rifting and salt deposition (discussion of Fig.
3), and implies a younger, offlapping salt architecture in
the central and western GoM and a younger, Callovian-
early Oxfordian salt along northwest Florida and north-
ern Yucatán. These implications cannot be tested yet
because strontium isotope ages have not been published
for salt samples from the distal offshore.
The second scenario dates salt deposition to
about the rift-drift transition (Fig. 4), but in the Batho-
nian rather than the often-cited Callovian-early
Oxfordian. The tectonic viability of this scenario is
afforded by initial sensitivity assessments using seismic
data (still underway) of the Equatorial Atlantic closure
reconstruction, which allow for the GoM to have been
roughly 200 km larger (north-south dimension) in the
Bajocian than is often thought. This scenario carries
additional primary implications, namely that sea-floor
spreading and initial deep water (abyssal) deposition
began in the Bathonian, some 8 m.y. earlier than in tra-
ditional GoM opening models. If so, rift-related heat
within the outer continental margins in earliest oceanic
crust would have had an additional 8 m.y. in which to
dissipate prior to Oxfordian source rock deposition,
helping to protect Oxfordian source rocks from early
burn out.
In either tectonic scenario, marine deposition
along the proximal rims of the GoM is not apparent
until the Oxfordian, delayed perhaps by isostatic
rebound, flexure, or thermal inflation around the basin
while the Norphlet and equivalent non-marine sands
were deposited. We suggest that deep-water salt deposi-
tion models (precipitation from hypersaturated solution
as in the Dead Sea today) need to be considered as via-
ble for the GoM and possibly elsewhere, because, if
feasible, they might explain the presence of thin salt
above little-faulted base-salt unconformities at the
depth of oceanic crust as a possible alternative to the
outer marginal collapse or the depth-dependent stretch-
ing hypotheses.
Acknowledgments
We are grateful to Ed Haire, Ken McDermott,
Rod Graham, Andy Pulham, Frank Peel, John Snedden,
Mark Rowan, Mike Hudec, Bob Erlich, and Wendy
Hale-Erlich for feedback and discussion on various
aspects contained in this paper. We thank Norm Rosen
and Carl Fiduk enormously for reviewing the manu-
script and improving its clarity and comprehension. The
first author is grateful to BHP, Chevron, Conoco-Phil-
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lips, ENI, Equinor, ExxonMobil, Hess, and Shell for
supporting the Cordilleran Research Program (Gulf of
Mexico) by Tectonic Analysis.
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Figure 1. Locations of features mentioned in text, plotted on relief geological map of Mexico (INEGI, 1980), and Getech free-air gravity image offshore
(from cover sleeve of James et al., 2009; red is positive, blue negative). Trace of extinct GOM spreading center after Pindell et al. (2016). Outline of oce-
anic crust (green line, dashed where approximate) according to mapping of ION seismic reflection database (unpublished). Salt and Smackover onlap
limits in northeast GoM after Dobson and Buffler (1997). Salt ages from A. Pulham (pers. comm., East Texas Basin), Snedden et al. (2018; Champion
well), Land et al. (1988; Pensacola, Florida), and Pindell et al. (in press 2019; others). HMA, FMA, and CMA are the Houston, Florida, and Campeche
magnetic anomalies, respectively. ACF, Angelina-Caldwell Flexure; WA, Wiggins Arch. Positions of Bajocian marine sections in Oaxaca after Sandoval
and Westermann (1986), and the position of the Florida Transfer Zone is approximated after numerous sources.
HMA FMA
Sunniland
Fields
Norton
Well
Salt limit
41
5
ACF WA
Louann Salt
Bajocian marine
Dated salt samples
1, Champion-Klepac-1
2, Pensacola, Florida
3, Hockley Mine, near Houston
4, East Texas Basin
5, Bacab-21, Campeche Basin
6, Mata Espino 101-B, Veracruz
Tuxpan
Huayacocotla
Embayment
6
3
2
Lower
Cretaceous
shelf edges
200 km
Smackover limit
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Figure 2. Comparison of the coastal positions of some Equatorial Atlantic reconstructions (Pindell, 1985; Pindell et al.,
2006; Moulin et al., 2010; Heine et al., 2013) that attempt to restore marginal extension and realign inferred limits of
prerift continental crust from each margin, plotted relative to a fixed African margin. The heavy black position of
northern Brazil is the “ION working fit”, employed in Figure 4.
African Reference Frame
Pindell et al. 2006
Moulin et al. 2010
100 km
Guinea
Plateau
Demerara
Plateau
100 km
Pindell 1985
Working ION fit (in progress)
Heine et al. 2013
Working ION fit gives a
looser GoM fit when
projected 700 km west
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Pindell et al. 86
Figure 3. 167 Ma reconstruction using the Equatorial Atlantic fit of Pindell et al. (2006), modified after Pindell et al. (in press 2019). This paleogeo-
graphic scenario implies an offlapping salt architecture in the central and western GoM, and salt deposition younger than 167 Ma in the eastern GoM.
Palinspastic shape of the northern Andes is modified after Dewey and Pindell (1986) and Pindell et al. (1998). Geological structure shown in Mexico
denotes a distributed, sinistrally transtensional mechanism for the southeastward expansion of continental crust into the area previously occupied by the
northern Andes, rather than employing the Mojave-Sonora Megashear of Anderson and Schmidt (1983). Paleolatitudes after Torsvik et al. (2012).
Abbreviations: A, Appalachian Front; O, Ouachita Front; M, Marathon Front; Sab, Sabine Uplift; HMA, FMA, and CMA are the Houston, Florida and
Campeche magnetic anomalies; ECMA, BSMA, and WAFMA are the East Coast, Blake Spur, and West African magnetic anomalies; G, Guajira; LM,
Lake Maracaibo; Trin, Trinidad region; MTFZ, PGFZ, Mejía-Talco and Pickens-Gilbertown fault zones; V, arc magmatism, Dem, Demerara Rise; TA,
en-echelon parts of Tamaulipas Arch; J, Sierra Juarez or Cuicateco Belt; MGA and Tampa, Middle Grounds and Tampa Arches; Apalach, Apalachicola
Basin; Chi Mass, Chiapas Massif; NA, SA, North and South America.
Evaporites
Red bed units (undiff)
Basement highs
Great
Bank
Blake
Plateau
Guinea
Colorado
Plateau NA/SA
plate
separation
direction
NORTH
AMERICA
Sab
Llanos
Wiggins
A
O
3
4
SOUTH
AMERICA
Trin
NA/SA
plate
separation
direction
Tampa
MGA
1
2
v
v
v
v
v
v
167 Ma, Bathonian Reconstruction (tighter fit)
GoM Evaporites
1 Champion-Klepac-1
2 Pensacola
3 Hockley Salt Dome
4 East Texas Basin
5 Bacab-21
6 Mata Espino 101-B
Hu Huehuetepec (not Sr dated)
LP La Popa (not Sr dated)
Dem
Caborca
Sabinas
LP
6
Hu
M
v
v
v
v
G
LM
Pre-Andean shape of NW South
America modified after Dewey and
Pindell (1986) and Pindell et al. (1998)
?
J
Farallon/NA
convergence
direction?
Bahamas-
Demerara Rise
Hotspot
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Pindell et al. 87
Figure 4. 167 Ma reconstruction using the looser “working ION” Equatorial Atlantic fit of Figure 2. This paleogeographic scenario suggests salt deposi-
tion was completed in the Bajocian, and that seafloor spreading began in the Bathonian. Palinspastic shape of the northern Andes modified after Dewey
and Pindell (1986) and Pindell et al. (1998). Geological structure shown in Mexico denotes a distributed, sinistrally transtensional mechanism for the
southeastward expansion of continental crust into the area previously occupied by the northern Andes, rather than employing the Mojave-Sonora
Megashear of Anderson and Schmidt (1983). Paleolatitudes after Torsvik et al. (2012). Abbreviations: A, Appalachian Front; O, Ouachita Front; M,
Marathon Front; Sab, Sabine Uplift; HMA, FMA, and CMA are the Houston, Florida, and Campeche magnetic anomalies; ECMA, BSMA, and
WAFMA are the East Coast, Blake Spur, and West African magnetic anomalies; G, Guajira; LM, Lake Maracaibo; Trin, Trinidad region; MTFZ,
PGFZ, Mejía-Talco and Pickens-Gilbertown fault zones; V, arc magmatism, Dem, Demerara Rise; TA, en-echelon parts of Tamaulipas Arch; J, Sierra
Juarez or Cuicateco Belt; MGA and Tampa, Middle Grounds and Tampa Arches; TP, Tuxpan Platform; Apalach, Apalachicola Basin; Chi Mass, Chi-
apas Massif; NA, SA, North and South America.
Colorado
Plateau
Caborca
Tampa
Great
Bank
Appalach
NA/SA
plate
separation
direction
167 Ma, Bathonian Reconstruction (looser fit)
Blake
Plateau
AFRICA
Sabinas
SOUTH
AMERICA
NORTH
AMERICA
1
3
Sab
4
V
V
1
2
v
Trin
Llanos
NA/SA
plate
separation
direction
Wiggins
Guinea
Dem
A
O
M
v
v
v
v
v
Pre-Andean shape of NW South
America modified after Dewey and
Pindell (1986) and Pindell et al. (1998)
GoM Evaporites
1 Champion-Klepac-1
2 Pensacola
3 Hockley Salt Dome
4 East Texas Basin
5 Bacab-21
6 Mata Espino 101-B
Hu Huehuetepec (not Sr dated)
LP La Popa (not Sr dated)
MGA
v
v
v
v
6
LM
G
vJ
Evaporites
Red bed units (undiff)
Basement highs
Farallon/NA
convergence
direction?
Bahamas-
Demerara Rise
Hotspot
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Figure 5. Three possibilities for how thin salt might occur at a distal continental margin upon a little-faulted base-salt
unconformity at the same structural level as adjacent the oceanic crust, as it does in the eastern GoM: (A) outer mar-
ginal collapse (Pindell et al., 2014); (B) mantle dynamic support with possible shallower-than-normal onset of seafloor
spreading; and (C) deep-water salt deposition (Dead Sea analogy).
2.6 km
Cont Crust
cont mantle
ocean
mantle
S.L.
cont mantle
Cont Crust
A, Outer marginal collapse between rift and drift?
Cont Crust
S.L.
Cont Crust
Cont Crust
dynamically supported,
with later dynamic subsidence
B, Begin seafloor spreading near sea level?
C, Deep-water salt deposition before break up?
Cont Crust
S.L.
0.7-2.6 km
Cont Crust
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