The Gulf Trough/Suwannee Strait in the subsurface of the Georgia Coastal Plain

1
The Gulf Trough/Suwannee Strait in the subsurface
of the Georgia Coastal Plain
2
Trough is a term that a geologist interested in
sedimentary rocks would use. A trough in this
sense is an elongate basin that could be or has
been filled with sediment. The shape of that
body of sediment (or sedimentary rock, after it
became lithified) would be very distinctive.
3
The Tongue of the Ocean is a deep-water feature
that dissects the Great Bahama Bank east of
Andros Island. Because of its shape it is
potentially a sedimentary trough. If you imagine
filling it with sediment, that body of sediment
would have a distinctive overall shape not one
like a delta (wedge) or a lake (lens) or a desert
dunefield (sheet).
Tongue of the Ocean
4
The shape of sediment fill in a trough is
comparatively long in one horizontal dimension.
Fill in the Tongue of the Ocean would be elongate
in a NNW/SSE direction. In both the other
horizontal dimension, at a right angle to the
long one, and in terms of sediment thickness (the
vertical dimension) the sediment body would be
shorter and thinner. Fill in the Tongue of the
Ocean would be narrow in a ENE/WSW direction and
roughly as thick as it would be wide. In
cross-section (along the short line on the map)
it would look something like the diagram below.
short dimension
Sediment Fill
long dimension
5
Stratigraphers therefore have two ways to
recognize an ancient trough, like the Gulf
Trough. We can recognize the basin itself or
recognize the distinctive shape of the basin
fill. To show you how we see and illustrate
these things, first I need to take you on a short
side trip. First, consider the symbolism of the
Contour lines on a topographic map. Each
contour connects Points of equal elevation,
enclosing higher elevations and excluding lower
ones. Closer spacing Steeper ground. Highest
point on a hill Is in the center of the center
contour line.
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YOUNGER OLDER
Second, remember that the rocks deposited to form
the Coastal Plain range in age from Triassic to
Holocene. The Gulf Trough has persisted for much
of that time, eventually filling entirely some
time after the Oligocene. Time units are usually
represented in a vertical column, oldest at
bottom, because that is the way we expect them to
stack as they accumulate
7
Geologists use the same contour line symbolism
for other values than modern land surface
elevation. The map below uses them to illustrate
the elevation of the top of a particular rock
body (whatever is below the lowest Cretaceous
rock) buried under the Coastal Plain. Negative
values indicate a depth below sea level. The
values for this map come from measured depths
to this surface in deep water and oil test
wells. This sort of map is called a
structure contour map. The surface has
a pronounced trough in deep southwest GA, with
its greatest depth right at the southwest corner
of the state.
(from Herrick and Vorhis, 1963)
8
Though the axis of the trough shifted as it
filled, the two structure contour maps below show
that it persisted through the end of the Eocene
(left -- 34my) and the end of the Oligocene
(right -- 23my). This persistence probably
resulted from both the time required to fill such
a deep hole and continued subsidence of the
trough as it filled.
(both from Herrick and Vorhis, 1963)
9
Geologists use the same symbolism for other
values than elevations. The map below uses
contour lines to illustrate the total thickness
of a particular rock body (the Lower Cretaceous
package) buried under the Coastal Plain. The
values for this map come from measured
thickness in deep water and oil test
wells. This sort of map is called an
isopach map. The rock body has partially
filled the trough on the pre-Cretaceous surface
shown in a previous slide.
(from Herrick and Vorhis, 1963)
10
In the Late Cretaceous, and probably in the
Paleocene, the isopach maps and other evidence
points toward a different orientation for the
trough. The map below at right summarizes the
various thickness patterns seen in the Coastal
Plain rocks.
(from Herrick and Vorhis, 1963)
11
(from Herrick and Vorhis, 1963)
12

Isopach map
Structure contour map
Miocene to Recent fill of
an end-Oligocene trough.
(from Herrick and Vorhis, 1963)
13
That is the evidence for a Gulf Trough. What is
the Suwannee Strait?
14
Strait is a term a geographer would use to
identify a narrow body of water connecting two
larger bodies of water. Usually when I think
about this thing I think about it as a
paleogeographer. Straits are also frequently
fairly deep as well as being narrow. They
frequently allow a current to flow from one large
water body to the other. The deeper water in the
strait, the scouring potential of any current
that is present, and the differences in current,
wave, and tide conditions farther away from the
strait often mean that sediment types occur in
bands parallel to the strait edge. Those
sediment types recur along straits of different
ages.
15
The Florida Straits connect the Gulf of Mexico to
the Atlantic. The northern branch of the Florida
Straits carry the main body of the Gulf Stream
out of the Gulf and into the Atlantic.
Gulf Stream
FLORIDA STRAITS
16
Florida Bay Lime Mud
Lower Keys
Marathon
Hawk Channel Lime Mud
?
White Banks Lime Sand
Key West
?
Florida Straits
Rhodolith Gravel
Reef Tract
?
?
17
Two common members of the red algal community
that lives in the rhodolith gravel below the reef
tract in southern Florida.
Red algal rhodolith from Florida (Recent)
Meoma ventricosa from White Banks, Florida
(Recent)
18
Distribution of the Bridgeboro Limestone, a
rhodolith-rich Oligocene rock.
(Huddlestun, 1993)
(Manker and Carter, 1987)
19
A cross-section of the Oligocene and younger
strata in the Gulf Trough in southwestern
Georgia. (Huddlestun, 1993)
20
Two common members of the red algal
paleocommunity that occurs in the Bridgeboro
Limestone (Oligocene) flanking the Suwannee
Strait in southwestern Georgia.
Archaeolithothamnium sp. Macropneustes
mortoni (one cut to show characteristic internal
structure)
21
Recent (Florida) Oligocene (Georgia and Florida
22
A Paleocene rhodolith from the algal member of
the Clayton Limestone
23
Also occur 120km almost due west at Rutledge, AL
24
The first indication of the existence of a
Suwannee Strait came from the work of two
paleontologists (Ester and Paul Applin)
interested in Foraminifera tiny protozoans with
an excellent fossil record. Their size insures
that they can be taken whole from well samples,
and so they are very well studied by petroleum
geologists. The bulk of the Applins evidence
for a strait was the disjoint occurrence of
sediment types and the apparent distinction of
the types of forams on either side of the
strait. Straits are often sites of biotic
disjunction boundaries between two biotic
provinces because they block migration.
25
One of the key observations that led A.R.
Wallace to his evolutionary ideas was the
Wallace Line. At approximately the Sunda
Strait in Indonesia the typical Asian biota ends
and the Australian biota replaces it.
26
Of course Wallace was studying land animals and
plants, and it makes sense that a strait should
hinder their movements. Forams are marine, so
why a seawater barrier should stop them is not
very obvious. Still, many types of marine fossil
organisms seem to be different on either side of
the Suwannee Strait. In the early 90s I wanted
to know why.
27
Clustering locations based on the sea urchin
species of middle Priabonian age that occur at
them, most of the peninsular Florida samples
group together (except Ocala). These are coded
blue on the diagram. Ocala is in green to show
its different cluster membership. All of the
Georgia samples occur in three somewhat different
groups. These are coded in pink.
28
3 of the Georgia groups actually link more
closely to the primary Florida group (black
circles) than to the primary Georgia group (black
squares).
More similar to each other than any is to main
Georgia cluster.
4 clusters link at this node.
29
The real reason for the difference in organisms
are the types of sediment for them to live in!
30
We concluded that the Suwannee Strait was not an
effective barrier to marine migrations. In rocks
of each small slice of time we examined either
the fossils were the same, the sediment types
didnt match, or one side of the Strait had too
few fossils to compare effectively.
31
Where did the Suwannee Strait come from?
32
Origins of the Suwannee Strait
Triassic volcanoes?
x
x
Arden, 1974
Long, 1974
33
East African Rift Basin (eastern arm)
Mt. Kenya
Mt. Kilimanjaro
Indian Ocean
34
Chowns and Williams, 1983
35
There are numerous rift basins along the eastern
side of North America. They were active as this
continent first tried to rift from Pangaea, but
finally failed as the rifting shifted to what is
now the Mid-Atlantic ridge.
36
Cuba Disneyland Florida Straits
Suwannee Strait Reef Tract
Reef Tract (Pelham
Esc.) White Banks Dougherty
Plain