Magnetostratigraphy of the hominin-bearing Hadar Formation (Ledi-Geraru, Ethiopia), and regional evidence for environmental change ca. 3.2 Ma
Abstract
To date and characterize depositional environments of the hominin-bearing Hadar Formation, magnetostratigraphy was applied to Pliocene lacustrine sediments from the eastern part of the Hadar Basin (Ledi-Geraru research area). Characteristic geochemistry and lithologic features of three tuffaceous horizons at the bottom, middle and top of the stratigraphy are recognized as the Sidi Hakoma Tuff (SHT), Triple Tuff-4 (TT-4) and the Kada Hadar Tuff (KHT), respectively, previously dated by 40Ar/39Ar in other part of the basin. Demagnetizations and rock magnetic analysis of paleomagnetic samples collected at regular 50 cm intervals on a total stratigraphic thickness of 230 meters between the SHT and the KHT enable us to isolate paleomagnetic directions from a primary detrital remanent magnetization mostly carried by (titano-)magnetites basaltic in origin. These results indicate two paleomagnetic reversals bracketing a reversed polarity interval identified as the Mammoth event (chron 2An.2r). The average paleomagnetic direction, consistent with existing paleomagnetic data, indicates a post-3 Ma counterclockwise vertical-axis tectonic rotation (5-10 degrees) of the Hadar Basin and pervasive shallowing of paleomagnetic inclination (5-10 degrees) related to sedimentation and compaction. Ages of tuffaceous layers and of paleomagnetic reversals show excellent consistency between 40Ar/39Ar dating and the most up to date astronomically tuned polarity time scale. Linear interpolations indicate constant sediment accumulation rates (~90 cm/kyr) throughout the section, except for the uppermost part which shows a threefold increase between the top of the Mammoth 2An.2r chron and the KHT. Along with existing chronostratigraphic results from the Hadar Basin, we show that the steady-state deposition, taking place in an eastward tilting basin since ~3.4 Ma, was regionally disrupted ca. 3.2 Ma by a relatively short-lived but significant change in environmental conditions. This disruption may have resulted in evolutionary changes previously documented in Australopithecus afarensis.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMGP43C1490S
- Keywords:
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- 1145 Tephrochronology (8455);
- 1500 GEOMAGNETISM AND PALEOMAGNETISM;
- 1535 Reversals: process;
- timescale;
- magnetostratigraphy;
- 1540 Rock and mineral magnetism;
- 4910 Astronomical forcing