Samuel T. Peavy Department of Geology and Physics Georgia Southwestern State University

1. Detailed Architecture of theSub-Coastal Plain South Georgia Basin as Revealed by Geophysical Data Samuel T. Peavy Department of Geology and Physics Georgia Southwestern State University Americus, Georgia

2. A Plan for Today’s Talk: • Data and Approach • Results of Potential Field Analyses • Tectonic Tie-in

3. Data and Approach: Data Sets • Gravity Data (from USGS) • COCORP Georgia/Florida Seismic Lines • Pre-Cretaceous Well Data (Chowns & Williams, 1983)

4. Data and Approach: Data Sets – Gravity 18,968 data points

5. Data and Approach: Data Sets – Gravity Data were gridded and contoured using “Surfer” program

6. Data and Approach: Data Sets – Gravity Northeast North Central West Southeast South Central The data were sub-divided into Western, Central, Northeast and Southeast regions.

7. Data and Approach: Data Sets – Gravity Northeast North Central West Southeast South Central Each region was analyzed separately and then combined to provide the best overall result.

8. Data and Approach: Data Sets – Seismic COCORP SEGY files were used to aid in the interpretation of the data.

9. Data and Approach: Data Sets – Deep Wells Lithologic information from Chowns & Willams (1983) were used to guide interpretation

10. Data and Approach: Data Sets – Deep Wells Well Information Overlain on Gravity Data

11. Data and Approach: Potential Field Attributes Potential Field Attribute Analysis (PFA) Consists of 3 analysis methods: • Analytic Signal (Nabighian, 1972; 1984) • Tilt Angle (Miller and Singh, 1994) • Local Wavenumber (Peavy, 1997)

12. Data and Approach: Potential Field Attributes Analytic Signal Tilt Angle Local Wavenumber

13. Data and Approach: Potential Field Attributes

14. Data and Approach: Potential Field Attributes • What to look for in the maps: • Analytic Signal and Local Wavenumber will detect the edges of bodies, although the latter is more sensitive. • Tilt Angle will find the central location where the density contrast is locally greatest.

15. Results: West Georgia – Gravity Data with Wells and COCORP Lines 1 Metamorphic 2 Felsic/Inter. Igneous 3 Rhyolite/Tuff 4 Triassic Red Beds 5 Triassic/Diabase 6 Diabase 7 ?/Diabase 8 Pz/Tr/Diabase 13 11 12

16. Results: West Georgia – Analytic Signal Not much to see…

17. 1 Metamorphic 2 Felsic/Inter. Igneous 3 Rhyolite/Tuff 4 Triassic Red Beds 5 Triassic/Diabase 6 Diabase 7 ?/Diabase 8 Pz/Tr/Diabase Results: West Georgia – Tilt Angle Interesting Results! Lows seem to correlate with Triassic Materials. 13 11 12

18. 13 11 12 Results: West Georgia – Wavenumber Complex trends may establish the locations of border faults.

19. 13 11 12 Results: West Georgia Tilt Angle and Wavenumber seem to reveal some interesting features. Let’s compare them to some seismic data from the same area…

20. Results: Tilt Angle vs. GA-11 3,430,000 3,500,000

21. Results: Tilt Angle vs. GA-12 3,415,000 3,452,000

22. Results: Wavenumber vs. GA-11 3,430,000 3,500,000

23. Results: Wavenumber vs. GA-12 3,415,000 3,452,000

24. 13 11 12 Results: West Georgia In general, lower regions on the Tilt Angle correlates with basin depocenters, while Wavenumber indicates boundaries.

25. Results: West Georgia Since the goal is to get an idea of overall basin geometry, let’s focus on the Tilt Angle maps from the rest of southern Georgia.

26. Results: North Central Georgia – Tilt Angle 1 Metamorphic 2 Felsic/Inter. Igneous 3 Rhyolite/Tuff 4 Triassic Red Beds 5 Triassic/Diabase 6 Diabase 7 ?/Diabase 8 Pz/Tr/Diabase

27. Results: South Central Georgia – Tilt Angle 1 Metamorphic 2 Felsic/Inter. Igneous 3 Rhyolite/Tuff 4 Triassic Red Beds 5 Triassic/Diabase 6 Diabase 7 ?/Diabase 8 Pz/Tr/Diabase

28. Results: Northeast Georgia – Tilt Angle 1 Metamorphic 2 Felsic/Inter. Igneous 3 Rhyolite/Tuff 4 Triassic Red Beds 5 Triassic/Diabase 6 Diabase 7 ?/Diabase 8 Pz/Tr/Diabase

29. Results: Southeast Georgia – Tilt Angle 1 Metamorphic 2 Felsic/Inter. Igneous 3 Rhyolite/Tuff 4 Triassic Red Beds 5 Triassic/Diabase 6 Diabase 7 ?/Diabase 8 Pz/Tr/Diabase

30. Merged Tilt Angle Map

31. Cross Strike Trends?

32. Generalized Basin Geometry Using Tilt Angle, Well and Seismic Information “Tifton”

33. Results: Tectonic Tie-In Exposed Mesozoic basins along the Applachians follow a pattern that seems to reflect preexisting structures. From Costain and Çoruh (1989)

34. Results: Tectonic Tie-In The basins closely follow the trend established by rifting of the Laurentian Margin. Suture? (after Thomas, 1983)

35. Results: Tectonic Tie-In Rifting during the Mesozoic was oblique to the trend of structures in southern Georgia (after Thomas, 1983)

36. Results: Tectonic Tie-In Results of physical model experiments, such as those by McClay and White below, show that oblique rifting leads to complex fault systems with complex basin geometries such as seen here. From McClay and White (1995)

37. Results: Tectonic Tie-In Another possibility is that there may have been two-phase rifting, with initial orthogonal rifting followed by oblique extension. From Keep and McClay (1997)

38. “Tifton” Results: Tectonic Tie-In The combination of apparent cross-strike structures and complex, synthe tic and antitheticfault geometries could be the result of rifting oblique to major structures in south Georgia, or perhaps two phases of rifting (orthogonal followed by oblique).

39. Conclusions: • Analysis of potential field, well and seismic information reveal the complexities of the structure of the South Georgia Basin. • Tilt Angle proves particularly useful in combination with seismic and well data in establishing basin locations. • The complex geometry of the Basin was established by either oblique or two-phase rifting during the Mesozoic.