THE LARGEST WIND RIPPLES ON EARTH: REPLY

MILANA, J. P, FORMAN, S., AND KRÖHLING, D.

GEOLOGY

GEOLOGICAL SOC AMER, INC

Lugar: Boulder; Año: 2009 vol. 38

0091-7613

We thank de Silva the opportunity to discuss the origin of the unique megaripples (MR) found on the Puna. Forman and Kröhling were invited as we produced new data supplied here. De Silva suggests that the genetic relation inferred by Milana (2009) between MR and bedrock topography is in error. Next four topics may encompass de Silva concerns. 1) Age of MR: We do not agree with the assertion that all MR form in days or weeks. If they do, they are quite small as those 1 cm high described by Jerolmack et al. (2006) and Yizhaq et al. (2009). The latter showed those same MR attaining 0.7 m long and 4 cm high after a year. At Puna, wind reworked flat well locations into 1 m long and 10 cm high MR in 5 years (Fig.1). Bagnold (1941), suggested it may take decades or centuries to form large MR as they grow progressively slower. Puna MR reach 43 m long and 2.3 m high. We dated several giant MR by optically stimulated luminescence. An average MR (Fig. 1) gave an age of 1710 ± 130 yr (7.5 cm over bedrock) and 635 ± 45 yr (20 cm over bedrock, 22 cm under surface), although some MR are as old as 3 ka. De Silva used ignimbrite deflation rates to postulate a dissimilar rate of evolution between MR and ignimbrite morphology. Three main ignimbrites are associated to these MR, with different resistance to erosion: The Campo of Piedra Pómez ignimbrite (c. 73 ± 23 ka) is quite resistant due to its vapor-phase crystallization, whereas the Purulla (22.9 ± 8.6 ka) and El Médano (12,2 ± 8.6 ka) ignimbrites (Arnosio et al., 2008) are unconsolidated, and show deflation corridors of 20-25 m deep excavated in the Holocene. Even if deflation rates mentioned are from the harder ignimbrite, the time claimed by de Silva to excavate MR troughs is compatible to our findings, also proving Bagnold´s beliefs about large MR age. Thus, we do not see any controversy in encompassing the megaripple evolution and the ignimbrite deflation. 2) Structure: The bedrock-MR association is shown by two profiles (Fig. 1), measured with a topographic instrument. Six crests were excavated exposing profiles along all the granular part to check position of the ignimbrite. They show coarsening upward while each MR bed that slopes at c. 10º, is truncated upslope and fines downwind, downlaping on the ignimbrite. This is coherent with present surface: coarsest and heaviest clasts on the crest and a progressive fining along the lee face. This structure results from MR migration along with a net vertical accumulation over the locally bare ignimbrite surface, starting about 3000 yrs ago. The topographic survey shows an average depth of ignimbrite troughs of 1.5 m, probably excavated during MR evolution. These perpendicular troughs, only observed in association to MR, cannot be ascribed to other than aeolian action due to the (a) exact match between granular crests and troughs on the ignimbrite (see GoogleEarth at 26º37´S; 67º46`W), (b) trough orientation perpendicular and oblique to a significant local slope, precluding formation by flowing water and (c) their high regularity only comparable to mature linear bedform networks. 3) De Silva also questioned the creep of 2-3 cm sized volcanic clasts. Creep was assumed by Milana (2009) as the main transport mode of the Puna gravel in the smaller MR type present between giant MR. The action of saltating pumice clasts cannot be used to explain the creep of 1-3 cm denser clasts as many MR are formed in the absence of such pumice clasts. Besides, de Silva calculations for a pumice clast (0.8 g/cm³) at the initiation of movement (creep or reptation) seems insufficient to catalyze movement of 3-times heavier clasts, and less likely to push them 10º upslope until the MR crest. Replacing density in de Silva formulations with the correct clast density (2.42 g/ cm³), increases threshold friction speed by ~3 fold than estimated, suggesting that his scenario is unrealistic. 4) While creep action is not in debate, its linkage to MR final shape should be. The lack of correspondence between grain size and megaripple size does not accomplish the modified ballistic hypothesis. On the other hand, shape and size of MR seem controlled by topography as shown by Milana (2009, Fig. 3), suggesting that wind flow structure controls to some extent, large MR formation. However, this is hypothetical and will remain so, until instrumental data could be acquired. Concluding, we do not find any disagreement with data supplied by de Silva and the genetic relation between MR and bedrock topography. Thus, the most likely explanation to this way of bedrock erosion is the still unknown interplay between wind flow structure and a granular bed.