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

A Plan for Today’s Talk: 1. Data and Approach 2. Results of Potential Field Analyses 3. Tectonic Tie-in

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

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

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

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

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

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

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

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

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) Data and Approach: Potential Field Attributes

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

Data and Approach: Potential Field Attributes

What to look for in the maps: 1.Analytic Signal and Local Wavenumber will detect the edges of bodies, although the latter is more sensitive. 2.Tilt Angle will find the central location where the density contrast is locally greatest.

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

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

Results: West 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 Interesting Results! Lows seem to correlate with Triassic Materials.

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

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…

3,430,0003,500, Results: Tilt Angle vs. GA-11

Results: Tilt Angle vs. GA-12 3,415,0003,452,000

Results: Wavenumber vs. GA-11 3,430,0003,500,000

Results: Wavenumber vs. GA-12 3,415,0003,452,000

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

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. Results: West Georgia

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

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

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

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

Merged Tilt Angle Map

Cross Strike Trends?

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

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

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

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

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)

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)

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

Conclusions: 1.Analysis of potential field, well and seismic information reveal the complexities of the structure of the South Georgia Basin. 2.Tilt Angle proves particularly useful in combination with seismic and well data in establishing basin locations. 3.The complex geometry of the Basin was established by either oblique or two-phase rifting during the Mesozoic.