In the second
and third parts of this four-article Geophysical Corner series we
defined a kinematic framework for the evolution of the Gulf of Mexico
region by restoring Andean deformations and progressively closing
the Atlantic Ocean.
This month, we further evolve this framework to build a palinspastically
quantitative reassembly of continents and continental blocks that
were separated during the Mesozoic rifting and subsequent drift
in the Gulf of Mexico region -- key features of which are shown
in figure 1.
show primary developmental stages in the Gulf's evolution:
Post-Gulf formation (figure 2).
Post-salt/pre-seafloor spreading (figure
Early syn-rift (figure 4).
elements applicable to the reconstructions are as follows.
our Oligocene reconstruction of northern South America (article
two) is further modified for Late Jurassic and Cretaceous time by
removing island arc and other terranes that were accreted to in
the Late Cretaceous and Early Tertiary (shape portrayed in figures
2 and 3).
We can then estimate and restore Jurassic extension in the rift
basins of the Andes (using principles outlined in the August EXPLORER,
which gives us an Early Jurassic shape for the northern Andes that
can be closed against North America (figure
figures 2-4 show that the entire region of Florida, the Blake Plateau
and the Bahamas (and the "Cuban autochthon" beneath the
Cuban arc) were strongly controlled by fracture zone trends of the
In this region,
plate separation was achieved by NW-SE stretching of crustal elements
separated by transcurrent faults. Middle Jurassic basalt extrusion
was commonplace in zones of high stretching.
"corridor" between transcurrent faults underwent different
amounts of stretching and displacements relative to the others.
The conjugate margin to the Southern Bahamas flank is the transcurrent
margin of Guyana.
unlike the Florida region, the Yucatan Block moved independently
-- in two distinct stages -- of the larger continents as the Gulf
At the time of figure 4, there is only
a small range of paleo-positions in which Yucatan could have fit
geometrically without overlap of palinspastically restored (i.e.,
rift-related stretching removed) areas of continental crusts. This
position can be achieved by rotating present-day Yucatan clockwise
about "pole A" (figure 4),
which closes most of the Gulf by placing Yucatan snugly against
the northeast Mexico-Texas-northwest Florida paleo-margin.
does not, however, close the southeastern Gulf. There, the crust
of South Florida -- including that of the "Tampa Arch"
-- must be retracted northwestward against Yucatan and out of an
overlap position with Demerara Rise, off the Guyana margin.
Thus, the southernmost crustal corridor of the Bahamas must have
migrated SE, probably along our "Everglades Fracture Zone"
(figure 1) between the times of figures
3 and 4.
the geology of the eastern Mexican margin and the occurrence of
Louann and Campeche salt suggest that the Gulf opened in two stages.
The first, or syn-rift, stage -- between the times of figures
3 and 4 -- involved intra-continental
stretching between Yucatan and North America about "pole B1,"
and between Yucatan and South America about "pole B2,"
in figure 4.
This migration defined an arcuate transcurrent trend defined by
basement contours along the northern Tamaulipas Arch in south Texas.
It also created a sinistral shear couple in the Louisiana-Mississippi
area, which allowed for minor counterclockwise rotation of the Wiggins
and Middle Grounds arches (figures 1
and 4) and the associated formation
of the wedge shaped East Mississippi and Apalachicola salt basins
to the north of each, respectively.
This syn-rift stage about "pole B1" can be modeled satisfactorily
to Early Oxfordian time to achieve a good reconstruction of the
Louann and Campeche salt provinces flanking the central Gulf (figures
1 and 3).
In our modeling,
we see no need to invoke significant salt deposition on oceanic
crust in the Gulf. Also, during this stage, the southern Bahamas
crustal corridor migrated southeast in addition to undergoing internal
stretching -- hence, the Everglades fracture zone and the Guyana
marginal fault zone were both active at this time.
of Yucatan from its pre-rift to its present position requires that
eastern Mexico was a transform rather than a rifted margin. We consider
that Yucatan did not have the Chiapas Massif attached to it during
the syn-rift phase.
we cannot satisfactorily fit a combinedYucatan/Chiapas Massif
into the northern Gulf, especially when we reverse the effect
of Cenozoic shortening in Sierra de Chiapas.
we believe that the Chiapas syn-rift salt basin is best explained
by early transtension along a crustal scale fault beneath it.
second stage of Yucatan motion began about "pole C" of
figure 3, in the Early Oxfordian, at
the end of salt deposition.
stage of motion and its pole of rotation are constrained by:
data along the eastern Mexican margin, which show an abrupt NNW-SSE
trending ocean-continent boundary.
anomaly data in the eastern Gulf.
of the once-adjacent margins of the Louann and Campeche salt basins.
that the Chiapas Massif was picked up by Yucatan in this stage as
a consequence of the onset of seafloor spreading in the Central
Gulf -- and because the pole of rotation changed in Stage 2, the
orientation and position of transforms also must have changed. This
new phase of motion had a more southerly direction than the previous
ridge almost reached the Mexican coast and, hence, the new transform
along eastern Mexico would have picked up an additional wedge of
crust, which we believe is Chiapas Massif and which had been emplaced
there during the syn-rift phase by sinistral transcurrent motions
within greater Mexico.
As with the
Gulf of Mexico, the synchronous creation of the "Proto-Caribbean
Basin" also must have involved a rotational opening between
Yucatan and Venezuela-Trinidad.
2-4, we show the approximate flowlines along which this basin opened,
as well as a hypothetical geometry of its Jurassic rifted margins
-- now wholly overthrust by allochthonous Caribbean terranes.
of northern South America's and possibly eastern Yucatan's hydrocarbon
potential pertain directly to the geometries of these rifted margins,
such as the positions of marginal re-entrants that define differing
stratigraphic sequences due to differing subsidence histories.
Gulf kinematic model has some interesting implications for exploration.
the Eastern Mexican margin (unlike that of Texas) was a Jurassic
fracture zone in the north (Burgos-Tampico basins) and a transform
-- with active structuring until its Early Cretaceous death -- in
the south (Veracruz Basin).
subsidence history, occurrence of salt, distribution/thickness of
Late Jurassic source rocks and basement controls on future structural
development will all vary along strike along this margin due to
differing crustal properties and histories.
In the U.S.
Gulf margins, early syn-rift stretching was NNW-SSE until Early
Oxfordian times, but most of the stretching toward the end of this
phase occurred well offshore.
although salt deposition is generally assumed to be of Callovian
age, there is little evidence of open marine conditions in the Gulf
margins until upper Oxfordian (Norphlet-Smackover transition), and
thus salt deposition may have continued until Early Oxfordian.
Our Early Oxfordian
reconstruction accommodates known salt occurrence in the Gulf ("salt
fit"); hence, we consider that onset of seafloor spreading,
the change in the Yucatan-North America pole position, separation
of Louann and Campeche salt provinces, and initiation of open marine
conditions were nearly coeval and possibly causally related.
although the syn-rift stretching of the Florida Shelf region was
NW-SE, the extension direction in the deep eastern Gulf during stage
2 (seafloor spreading) was NE-SW about a nearby pole, such that
small circles (transform traces) should be arcuate and convex to
In Cuba, a
significant area of Bahamian crust was overthrust by Cuban arc assemblages
in the Paleogene. In the Jurassic, the southern Bahamian margin
(beneath Cuba) experienced sinistral strike-slip tectonics along
the Guyana margin of South America, followed by the eastward migration
of a Late Jurassic seafloor spreading ridge (Yucatan/South America
boundary) along the western half of the overthrust zone.
nature of this Jurassic margin should be considered in interpretations
of the Paleogene development of the Cuban thrust belt, Mesozoic
source rock paleogeography and oil migration pathways during Eocene
In the Proto-Caribbean,
the kinematics require westward-propagating Early and Middle Jurassic
rifting, followed by Late Jurassic seafloor spreading. The trends
of marginal re-entrants such as that defined by the Urica basement
transfer zone are defined by the first stage of Yucatan's motion.
passive margin section is predicted to have existed from the end
of Middle Jurassic, not Cretaceous as is commonly thought. A several
kilometer-thick, probable Late Jurassic shelf section in Eastern
Venezuela has not received much attention from exploration, and
the "Berriasian or older" salt in Gulf of Paria could
be Middle Jurassic (as is the salt in the Bahamas, Guinea Plateau
and Demerara Rise and Tacatú Basin).
Note the proximity of these areas on figure
In Sierra Guaniguanico
of western Cuba, the conjugate margin of Eastern Venezuela, the
lower Middle Jurassic San Cayetano strata indicate the existence
of a juvenile passive margin of that age, becoming fully marine
for Late Jurassic, as predicted here for Venezuela and Trinidad.
regional plate kinematic analysis is extremely cost-effective and
deserves an important role in the exploration of complex areas,
both early on and long-term.
The kinds of
implications we have drawn here also can be made from kinematic
analysis in other parts of the world. When applied properly to appropriate
areas, it is not arm waving.
Much can be
types and associated parameters such as early heat flow.
of regional reservoir-bearing depositional patterns.
ages of classes of structures, etc.
And all that
is gleaned can lead to the creation or dismissal of numerous play
an explorationist with a comprehensive kinematic framework available
to him or her will work more confidently -- and therefore, more
efficiently -- on nearly all other aspects of the exploration process.
frontier evaluation programs, regional kinematic analysis may not
tell you exactly where to drill, but it can often help to tell you
where not to drill.