Detailed Architecture of theSub-Coastal Plain

South Georgia Basin as Revealed by Geophysical Data

Samuel T. PeavyDepartment of Geology and Physics

Georgia Southwestern State UniversityAmericus, Georgia

A Plan for Today’s Talk:

1. Data and Approach2. Results of Potential Field Analyses3. Tectonic Tie-in

Data and Approach: Data Sets

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

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Data and Approach: Data Sets – Gravity

18,968 data points

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Data and Approach: Data Sets – Gravity

Data were gridded and contouredusing “Surfer” program

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Data and Approach: Data Sets – Gravity

The data were sub-divided into Western, Central, Northeast and Southeast regions.

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West

North Central Northeast

SoutheastSouth Central

Data and Approach: Data Sets – Gravity

Each region was analyzed separately and then combined to provide the best overall result.

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North Central Northeast

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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

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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

A x,y H x,y 2 Z x,y 2

Data and Approach: Potential Field Attributes

Tilt Angle

x, y tan -1 Z x,y H x,y

Local Wavenumber

x, y x,y

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Data and Approach: Potential Field Attributes

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.

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Results: West Georgia – Gravity Data with Wells and COCORP Lines

1 Metamorphic2 Felsic/Inter. Igneous3 Rhyolite/Tuff4 Triassic Red Beds5 Triassic/Diabase6 Diabase7 ?/Diabase8 Pz/Tr/Diabase

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Results: West Georgia – Analytic Signal

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Not much to see…

Results: West Georgia – Tilt Angle

1 Metamorphic2 Felsic/Inter. Igneous3 Rhyolite/Tuff4 Triassic Red Beds5 Triassic/Diabase6 Diabase7 ?/Diabase8 Pz/Tr/Diabase

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Interesting Results!Lows seem to correlate with Triassic Materials.

Results: West Georgia – Wavenumber

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Complex trends may establish the locations of border faults.

Results: West GeorgiaTilt Angle and Wavenumber seem to reveal some interesting features. Let’s compare them to some seismic data from the same area…

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Results: Tilt Angle vs. GA-11

Results: Tilt Angle vs. GA-12

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Results: Wavenumber vs. GA-11

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Results: Wavenumber vs. GA-12

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

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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.

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Results: West Georgia

Results: North Central Georgia – Tilt Angle

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1 Metamorphic2 Felsic/Inter. Igneous3 Rhyolite/Tuff4 Triassic Red Beds5 Triassic/Diabase6 Diabase7 ?/Diabase8 Pz/Tr/Diabase

Results: South Central Georgia – Tilt Angle

1 Metamorphic2 Felsic/Inter. Igneous3 Rhyolite/Tuff4 Triassic Red Beds5 Triassic/Diabase6 Diabase7 ?/Diabase8 Pz/Tr/Diabase

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Results: Northeast Georgia – Tilt Angle

1 Metamorphic2 Felsic/Inter. Igneous3 Rhyolite/Tuff4 Triassic Red Beds5 Triassic/Diabase6 Diabase7 ?/Diabase8 Pz/Tr/Diabase

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Results: Southeast Georgia – Tilt Angle

1 Metamorphic2 Felsic/Inter. Igneous3 Rhyolite/Tuff4 Triassic Red Beds5 Triassic/Diabase6 Diabase7 ?/Diabase8 Pz/Tr/Diabase

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Merged Tilt Angle Map

Cross Strike Trends?

Generalized Basin GeometryUsing 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-InThe basins closely follow the trend established by rifting of the Laurentian Margin.

Suture?

(after Thomas, 1983)

Results: Tectonic Tie-InRifting during the Mesozoic was oblique to the trend of structures in southern Georgia

Results: Tectonic Tie-InResults 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-InThe 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.