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. Tongue of the Ocean 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).

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. long dimension short dimension Sediment Fill

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.

6. Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement” 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) Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

8. (both from Herrick and Vorhis, 1963) 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. Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

9. (from Herrick and Vorhis, 1963) 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. Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

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) Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

11. (from Herrick and Vorhis, 1963) Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

12. (from Herrick and Vorhis, 1963) { Miocene to Recent fill of … … an end-Oligocene trough. Isopach map Structure contour map Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

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. FLORIDASTRAITS 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

16. Key West Florida Bay – Lime Mud Lower Keys Hawk Channel – Lime Mud White Banks – Lime Sand Florida Straits Reef Tract Rhodolith Gravel Marathon ? ? ? ? Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

17. Red algal rhodolith from Florida (Recent) Meoma ventricosa from White Banks, Florida (Recent) Two common members of the red algal community that lives in the rhodolith gravel below the reef tract in southern Florida.

18. (Huddlestun, 1993) (Manker and Carter, 1987) Distribution of the Bridgeboro Limestone, a rhodolith-rich Oligocene rock. Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

19. A cross-section of the Oligocene and younger strata in the Gulf Trough in southwestern Georgia. (Huddlestun, 1993) Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

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) Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

21. Recent (Florida)Oligocene (Georgia and Florida Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

22. A Paleocene rhodolith from the algal member of the Clayton Limestone Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

23. Also occur ~120km almost due west at Rutledge, AL Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

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. Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

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 90’s 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. Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

28. 4 clusters link at this node. 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. Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

29. The real reason for the difference in organisms are the types of sediment for them to live in! Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

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 didn’t 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 Arden, 1974 x x Triassic volcanoes? Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement” Long, 1974

33. Indian Ocean East African Rift Basin (eastern arm) Mt. Kenya Mt. Kilimanjaro Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

34. Chowns and Williams, 1983 Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”

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 (~Pelham Esc.) White Banks Dougherty Plain Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene U. Cretaceous L. Cretaceous Jurassic Triassic “basement”