The age of the largest wind ripples on Earth

MILANA, J. P., FORMAN, S, KROHLING, D.

Mendoza

Congreso; International Sedimentological Congress 2010; 2010

International Association of Sedimentologists

While wind ripples may form in minutes, their further evolution into larger bedforms is still a matter of debate. It is well known that, megaripples can evolve from a ripple field if grains coarser than sand are present in the bed. While its initial stage may respond to a pseudo-ballistic process, evolution into a larger bedform may not. Part of the answer to this quest relies on the age of the largest megaripples found. This would also help understanding the even larger megaripples observed in Mars surface. According to the pioneer work of Bagnold (1941) large megaripples may take centuries to form, and he noted also that they tend to grow progressively slower, suggesting their size is proportional to the square root of their age. The largest megaripples found up to now on Earth are from the Puna plateau (Milana, 2009) and hence, they are well suited to investigate their age. The giant megaripple field contain bedforms reaching up to 43 m in wavelength and 2.3 m in amplitude, while average wavelength is 24 m (n=200) and amplitude is 1.8 m (n=30). We dated by optically stimulated luminescence 4 single giant megaripples, and 2 smaller megaripple complexes. We also took a second sample in one megaripple in order to learn if the bedform was formed in one pulse, or if it was continuously evolving. The age of the single giant bedforms was determined dating the layers near their base. The age obtained for megaripple initiation along profile 1 was between 1,600 and 3,300 yrs, while megaripples along the profile 2, gave ages between 800 and 1,060 yrs. One large megaripple of 2.4 m amplitude but with only 50 cm thick gravelly foreset was dated at two levels giving 1,710 ± 130 yrs and 635 ± 45 yrs at 22 cm below surface. This megaripple suggests a steady vertical growth rate of 0.22 mm/yr, although rates would change between 0.5 to 0.13 mm/yr. As the dated foreset incline c. 10º, the expected average migration rate for these bedforms would be 1 to 3 mm/yr. The difference of ages obtained between profiles 1 and 2, is probably related to their position within the field as profile 1 is central, while profile 2 is marginal. This suggests that the entire megaripple field started to evolve at about 3,000 yrs and it is still slowly evolving, suggesting they are active bedforms. Another dates obtained from smaller megaripple complexes, which are the accretion of several smaller bedforms, gave ages around 3,000 yrs. Although they do not serve to define individual megaripple age, they support the conclusion that most megaripples today visible in this area started to evolve at about 3 Ka. These findings suggest that Bagnold’s interpretation of megaripples was quite certain as they may take centuries or even millennia to evolve. However, a more important conclusion is the fact these bedforms seem to be active nowadays, although their migration rate would be very slow. Thus, it is expected that a detailed study of the wind flow structure over these active bedforms could help to understand the wind dynamics required to shape these rare bedforms on Earth, although much more spread on other planets. Our findings also suggest that large megaripples are so rare on Earth because they require stable wind flow conditions and adequate sediment supply, which in this case, seems they were maintained during the last 3,000 yrs.