2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 136-1
Presentation Time: 9:00 AM-6:30 PM


YOHLER, Ryan Michael, Geological Sciences, Indiana University - Bloomington, 1001 E 10th Street, Bloomington, IN 47405, NJAU, Jackson K., Geological Sciences, Indiana University, 1001 E. Tenth Street, Bloomington, IN 47405 and BROPHY, J.G., Department of Geological Sciences, Indiana University, Bloomington, 1001 East 10th Street, Bloomington, IN 47405, ryohler@indiana.edu

Understanding the spatial and temporal distribution of the beds within the Olduvai Gorge basin is crucial in understanding the geologic history of an area that could lead to new insights about hominin evolution. Until recently, scientific study of the beds have been limited to what is exposed in outcrops within the gorge itself. The purpose of this study is to apply the first use of geophysical techniques to understand the subsurface in and around Olduvai Gorge. An EG and G Germet Portable proton magnetometer was used to create line profiles over Granite Falls to the west and all five major faults, which are numbered from east to west as First, Second, Third, Fourth, and Fifth Faults. The areas studied here targeted the basalt in Bed 1 across the faults and the Naabi Ignimbrite at granite falls.

When taking magnetic measurements of a magnetic body that has been faulted, the values produce a distinctive anomaly wave pattern when moving from hanging wall to footwall and vice versa. The data indicate that the Bed I basalt layers are faulted along the First, Second, Third, and Fifth Faults. Although the basalt is seen being faulted at the Fourth Fault in the gorge, the lack of the signature anomaly pattern could be due to the relatively small offset of the Fourth fault in comparison to the other faults. The Naabi Ignimbrite could not be distinguished from basement rock where it is exposed at granite falls. Whenever a magnetic survey moves over two bodies with different susceptibilities, a bell curve anomaly pattern is produced in the data. Since this doesn’t exist between the granitic basement and ignimbrite, the magnetic properties of the ignimbrite are not distinguishable from the basement and provides evidence that the data collected is from the Bed I basalt. Depth estimates and other filtering and smoothing operations will provide a clearer spatial picture of the basalt. This could redefine the ages for faults where basalt is not exposed, such as the First and Fifth Faults. This leads to a better understanding of how and when the gorge formed.